Just FYI:
While I had the target mountpoint directory opened in Windows Explorer I got the following error when trying to mount a tresor:
org.cryptomator.common.vaults.Volume$VolumeException: Unable to mount Filesystem
at org.cryptomator.common.vaults.DokanyVolume.mount(DokanyVolume.java:48)
at org.cryptomator.common.vaults.Vault.unlock(Vault.java:151)
at org.cryptomator.ui.unlock.UnlockWorkflow.attemptUnlock(UnlockWorkflow.java:100)
at org.cryptomator.ui.unlock.UnlockWorkflow.call(UnlockWorkflow.java:81)
at org.cryptomator.ui.unlock.UnlockWorkflow.call(UnlockWorkflow.java:44)
at javafx.concurrent.Task$TaskCallable.call(Task.java:1425)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.Executors$RunnableAdapter.call(Unknown Source)
at java.base/java.util.concurrent.FutureTask.run(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor.runWorker(Unknown Source)
at java.base/java.util.concurrent.ThreadPoolExecutor$Worker.run(Unknown Source)
at java.base/java.lang.Thread.run(Unknown Source)
Caused by: org.cryptomator.frontend.dokany.DokanyMountFailedException: Error while mounting.
at org.cryptomator.frontend.dokany.MountFactory.mount(MountFactory.java:130)
at org.cryptomator.frontend.dokany.MountFactory.mount(MountFactory.java:116)
at org.cryptomator.common.vaults.DokanyVolume.mount(DokanyVolume.java:43)
... 11 more
Caused by: com.dokany.java.DokanyException: DokanMain returned error code-5: Dokan mount failed - Cannot assign a drive letter or mount point. Probably already used by another volume.
at com.dokany.java.DokanyMount.lambda$mount$2(DokanyMount.java:98)
... 1 more
Once I closed Explorer mounting the tresor worked fine.
Looks like viewing the mountpoint in Explorer somehow locks the target directory such that mounting a tresor is inhibited.