Unable to open more than two vaults as directory

When having two vaults opened as directories and trying to open a third one, I just get an eror message saying details are in the log file.

This happens using version 1.4.15 (with Dokan 1.3.0.1000) on Windows 10, but I also tried a machine running Windows 7 with the same results.

Opening several vaults as drives works fine (up to five vaults so far).

Here is the log:

14:32:58.887 [main] INFO  org.cryptomator.launcher.Cryptomator - Starting Cryptomator 1.4.15 on Windows 10 10.0 (amd64)
14:32:59.214 [main] INFO  o.c.launcher.IpcProtocolImpl - Received launch args: 
14:32:59.728 [JavaFX Application Thread] INFO  org.cryptomator.launcher.Cryptomator - JavaFX application started.
14:33:00.036 [JavaFX Application Thread] INFO  org.cryptomator.jni.JniModule - loaded WinFunctions.dll
14:33:00.112 [JavaFX Application Thread] INFO  o.c.ui.controllers.MainController - Unable to setPreferencesHandler, probably not supported on this OS.
14:33:04.837 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:04.925 [JavaFX Application Thread] INFO  o.c.ui.controllers.WelcomeController - Current version: 1.4.15, lastest version: 1.4.15
14:33:09.343 [JavaFX Application Thread] INFO  o.c.ui.controllers.WelcomeController - Current version: 1.4.15, lastest version: 1.4.15
14:33:23.047 [JavaFX Application Thread] INFO  o.c.k.WindowsProtectedKeychainAccess - Unable to load existing keychain file, creating new keychain.
14:33:23.945 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:33.692 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:35.193 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:38.858 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:40.682 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:49.667 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:33:54.666 [Background Thread 1] INFO  com.dokany.java.DokanyDriver - Dokany version: 130
14:33:54.666 [Background Thread 1] INFO  com.dokany.java.DokanyDriver - Dokany driver version: 400
14:34:12.718 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:34:21.055 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:34:22.610 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:34:25.338 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:34:31.240 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:34:32.252 [Background Thread 1] INFO  com.dokany.java.DokanyDriver - Dokany version: 130
14:34:32.252 [Background Thread 1] INFO  com.dokany.java.DokanyDriver - Dokany driver version: 400
14:35:26.267 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:35:38.081 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:35:39.233 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:35:40.528 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:35:52.439 [pool-1-thread-1] INFO  o.c.common.settings.SettingsProvider - Settings saved to C:\Users\Aelg\AppData\Roaming\Cryptomator\settings.json
14:35:56.307 [Background Thread 5] INFO  com.dokany.java.DokanyDriver - Dokany version: 130
14:35:56.307 [Background Thread 5] INFO  com.dokany.java.DokanyDriver - Dokany driver version: 400
14:35:56.321 [Background Thread 6] WARN  com.dokany.java.DokanyDriver - Error while mounting
java.lang.IllegalStateException: Mount failed: Cannot assign a drive letter or mount point. Probably already used by another volume.
	at com.dokany.java.DokanyDriver.start(DokanyDriver.java:65)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
14:35:56.323 [Background Thread 5] ERROR o.c.frontend.dokany.MountFactory - Mounting failed.
java.util.concurrent.ExecutionException: java.lang.IllegalStateException: Mount failed: Cannot assign a drive letter or mount point. Probably already used by another volume.
	at java.base/java.util.concurrent.FutureTask.report(FutureTask.java:122)
	at java.base/java.util.concurrent.FutureTask.get(FutureTask.java:205)
	at org.cryptomator.frontend.dokany.Mount.mount(Mount.java:36)
	at org.cryptomator.frontend.dokany.MountFactory.mount(MountFactory.java:119)
	at org.cryptomator.ui.model.DokanyVolume.mount(DokanyVolume.java:51)
	at org.cryptomator.ui.model.Vault.unlock(Vault.java:115)
	at org.cryptomator.ui.controllers.UnlockController.lambda$didClickUnlockButton$0(UnlockController.java:494)
	at org.cryptomator.ui.util.Tasks.lambda$create$0(Tasks.java:33)
	at org.cryptomator.ui.util.Tasks$TaskImpl.call(Tasks.java:139)
	at javafx.concurrent.Task$TaskCallable.call(Task.java:1425)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: java.lang.IllegalStateException: Mount failed: Cannot assign a drive letter or mount point. Probably already used by another volume.
	at com.dokany.java.DokanyDriver.start(DokanyDriver.java:65)
	... 5 common frames omitted
14:35:56.323 [Background Thread 5] WARN  o.cryptomator.ui.model.DokanyVolume - Failed to mount vault into C:\Daten\Sonstiges. Is this directory currently accessed by another process (e.g. Windows Explorer)?
14:35:56.325 [JavaFX Application Thread] ERROR o.c.ui.controllers.UnlockController - Unlock failed for technical reasons.
org.cryptomator.ui.model.Volume$VolumeException: Unable to mount Filesystem
	at org.cryptomator.ui.model.DokanyVolume.mount(DokanyVolume.java:56)
	at org.cryptomator.ui.model.Vault.unlock(Vault.java:115)
	at org.cryptomator.ui.controllers.UnlockController.lambda$didClickUnlockButton$0(UnlockController.java:494)
	at org.cryptomator.ui.util.Tasks.lambda$create$0(Tasks.java:33)
	at org.cryptomator.ui.util.Tasks$TaskImpl.call(Tasks.java:139)
	at javafx.concurrent.Task$TaskCallable.call(Task.java:1425)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:515)
	at java.base/java.util.concurrent.FutureTask.run(FutureTask.java:264)
	at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1128)
	at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:628)
	at java.base/java.lang.Thread.run(Thread.java:834)
Caused by: org.cryptomator.frontend.dokany.MountFailedException: java.lang.IllegalStateException: Mount failed: Cannot assign a drive letter or mount point. Probably already used by another volume.
	at org.cryptomator.frontend.dokany.MountFactory.mount(MountFactory.java:128)
	at org.cryptomator.ui.model.DokanyVolume.mount(DokanyVolume.java:51)
	... 11 common frames omitted
Caused by: java.lang.IllegalStateException: Mount failed: Cannot assign a drive letter or mount point. Probably already used by another volume.
	at com.dokany.java.DokanyDriver.start(DokanyDriver.java:65)
	... 5 common frames omitted

I cannot reproduce this on Windows 10 1809. Did you checked that

  1. the directories are really empty (also no hidden files like an .ini)
  2. all vaults to be opened are using different directories

What does your settings file look like for the vaults you want to open? (You can find it under %appdata%/Cryptomator/settings.json)
Every vault with a directory as a mount point should have the options "usesIndividualMountPath": true and "individualMountPath": "X:\\path\\to\\empty\\directory"