I can unlock my vault and edit the files as expected. However, when clicking on lock in the desktop app, nothing happens. Similarly, when closing the App and clicking on Lock and Quit, nothing happens either.
The log message means that there are still open file handles (at least one file in the vault is still open in a program) and because of this the locking of the vault is aborted (that there is no loss of change).
There is already an issue at Github about how to illustrate in Cryptomator that programs still accessing files: