[stumped]

malwarebytes free does not actually "run" , normally- its just waits to be run manually as an on demand process.
malwarebytes paid gets worse by the day. they spent months in denial then have failed to fix bad audio caused by web protection module adding latency spikes, and now there are many reports of it failing with win 2004. i decided to move on, even though I had 2 lifetime keys.. i almost was panicked into buying a new PC as pristine audio is very important to me and I'd spent a couple of weeks blaming and swapping around other things.

back to the problem - OP has tried lots of devices. time to try all the different ways there are to ping/ connect to a calibre server and see if that narrows it down any.

and also, change the assigned port.
there is no need to change the default port just because plex server is running, the plex default is 32400. mine runs on that and calibre runs on i 8080
if you are out of other options, try going back to the default ports.