Here is a possible solution:
Go to the folder where you stored the vault.
In my case, I stored it in OneDrive. (Yours could be iCloud, dropbox, etc.)
Look at the vault folder and check if it has a little cloud icon with a down arrow.
This means that the files are NOT downloaded on your Mac.
They are still on the cloud.
For Cryptomator to work, all the files in the vault must be downloaded to the Mac.
So you need to go into the folder and click on all the little “down-arrow cloud icons” to download everything to your Mac.
Once you do this, your vault with Lock and Open with zero problems.
It will do it so fast you won’t know what happened.
Hope this helped you!