This message indicates a system configuration error. Search our forums and the internet for “System error 53 The network path was not found”.
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To understand the problematic a little better: With selecting WebDAV Cryptomator is starting a local-only server, which is used to operate on an unlocked vault. The unlocked vault is integrated into the Windows Explorer with the net use command.
A VPN, a proxy server or security software can inhibit this on different ways (e.g. blocking access to localhost). A good overview from Windows is Redirecting.
Using WebDAV has some drawbacks, for example it is slow in general and the file size is restricted to 4GB in Windows (Windows: 4 GiB file size limit on virtual drive)