Sharing folders broken after installing MS 365

I have the Windows 11 arm version running on UTM (on Apple silicon MacBook), and am now facing a weird problem with shared folders. It works perfectly until I install Microsoft 365 on it - after which the shared folder disappears and the Spice Webdav service cannot be started (error 1053) in Windows, neither automatically nor manually. I have the impression that another service or something blocks the port for webdav (which should - please correct me if I am wrong - be port 9843). Using netstat -bano in Windows I do not find anything using that port. Can this be the problem? Does anyone have an idea why this problem occurs after installing Microsoft 365 (reproducibly, and it goes away after restoring the system from a Windows restore point made immediately before installing MS365)? Any ideas how to solve the problem???