To shed some light into this: The cause for this error is a misunderstanding/insufficient translation (-;
The text entered into the text field is used as a path to point to some directory or free volume. And since a single letter is not a valid path, you can’t unlock the vault. This option is for mounting your cryptomator vault into an empty directory, if you want to. The original english text of the Checkbox is “Use Custom Mount Point” and a mount point can be anything as long it is a valid (absolute) path to an empty directory