Hello everyone,
The problem itself has not yet been properly solved, but here is at least a workaround:
If I log in to the Nextcloud MacOS app via the IP address (with http) instead of via the domain with SSL certificate, the virtual files work perfectly. See also: [Bug]: macOS Virtual files VFS not working on Nextcloud 3.15.3 路 Issue #7979 路 nextcloud/desktop 路 GitHub
Note: Even if the problem is not really solved, I have simply marked the post as solved because it works via the IP address.