Klauss, a firewall on your Server or Client or both is blocking MC transactions between the Server and the Client. You need to investigate your firewalls and explicitly allow MC programs to operate. There is a small chance a Virus Scanner is the actual culprit if yours uses heuristics or similar, but I would look to the firewall first.
This explains perfectly the scenario of deleting a file on the Client, having it disappear on the Client View, but then reappearing when the Client is closed and reopened. i.e.
Delete file in Client.
The local copy of Server Library is updated with the deletion, removing the file from the view. This is immediately apparent in Theatre View, but requires a refresh of the View in Standard View, possibly.
Firewall blocks the deletion from being executed on the Server and also blocks the update of the Server Library. Probably the Server Firewall, but could be the Client Firewall.
The Client is closed.
The Client is reopened and a new, fresh copy of the Server Library is downloaded, which includes the "deleted" file.
Frustration and confusion ensue. ?
When you upgraded from MC21 to MC23 all the executables changed, so a firewall would want to be told those executables were okay to Allow again. If you have notifications turned on in your Firewall it should have generated messages asking whether you want to allow the programs access the first time you used MC23. Typically there are several notifications, for different parts of MC. I usually get two or three when I install a new major version, at times like initial connection to the Server, first playback of video, audio, etc. which require access to the internet to download madVR, LAV, codecs, etc. I'm using Norton 360 on all my Windows PCs and do not have it set to Silent Mode specifically so that I can see when the firewall needs to Allow access to something.
Check which MC21 programs are Allowed in your Firewall, and make sure that the equivalent MC23 programs are also Allowed. on your Server and Client.