[stumped]

ok - not what I expected but progress nevertheless
what devices do you have available to test with ?

maybe change the port from the default 8080 in case that is clashing with something
not sure HOW to do that but its probably just a preference setting
I would also consider , counterintuitively, setting an access password then connecting with the wrong password - to see if it really is connecting and checking

this is all being done wired- via LAN? no wifi involved ?- and no complicated subnets e.g. server is on something like 192.168.0.xx and client is on 192.168.0.yy ?

and circling back - if you set the calibre content library folder to shared, you can actually get to that and copy over a book via windows by navigating down into the author subfolders ?

at some point, if none of that helps, I think backup then nuke all of the calibre config folder, let that rebuild with all default values, then retry content server. Maybe there's a stuck , messed up cache somewhere that calibre is not auto clearing when it is stopped and restarted

do you have ANY internet security / Av type programs running that could be monitoring/blocking?
i doubt they are the cause but if you can eliminate them by suspending & re-testing or by running windows in safe mode, that may help by narrowing things down