Is WebDAV local or remote? Seems like there’s a connectivity issue. You could try some network debug depending on your skill. For example, does a ping test show packet loss? Are speeds good or too slow?
Duplicati.CommandLine.BackendTester.exe pointed to an empty folder is one way. Get starter URL from Export As Command-line and edit folder. Manual test can use Duplicati.CommandLine.BackendTool.exe