Phew 
Glad that it worked.
Do you mean with “open an existing vault” adding the vault to Cryptomator? Regarding the mount problem: If you are using Dokany ( look in the virtual volume tab in the general settings), this message is shown when you try to access the vault with admin priviliges. For more info, look at the topic Cryptomator on Windows: Accessing your vault with admin priviliges.
An alternative to Dokany is WebDAV. It works quite stable, but has some Windows specific drawbacks (due to its really bad implemenation) like that the file size is limited to 4GB, it is slow, filepath limit of 260 characters…