Now I found out what causes this above mentioned problem:
Whenever I take the mark away from “Show Drive” in the advanced options and then directly klick on “Unlock Vault” it is not possible to lock the vault again.
When I close the program after taking of the mark, restart the program, unlock a vault and then klick on “Lock Vault” everything works fine. It happens only in the case that I take the mark off “Show Drive” in the Advanced Options and unlock a vault directly after that.
I checked it now for some times, even reinstalled the program, checked again; it is always the same error message and the same behaviour of the program as mentionened above.
Xaver