Windows Explorer - Can't access files in synced folder

After installing and running nextcloudpi (26.0.3) inside my network, I installed the windows 11 client (3.10.0 after several updates) for synchronisation. I choose a complete folder sync with a folder on my local partition and started syncing.

The clients now stats, sync done. When I try to open files in my local folder, I get the error 0x8007016A - cloud provider not supported / running. Checking the state of the files, windows show the status sync pending. I tried to tick the “always on device” (I don’t know the correct wording of this setting as my windows is a german version) setting, but didn’t change the behaviour.

I have no idea, what to do…

Thanks for any advice, Ralph!