I encountered this last night, firing up PSO on my wife's laptop. (It had been several months.)
I'd solved it before by adding it to the "Exclude from scan" folder in Windows Defender.
But for whatever reason, Windows likes to occasionally quarantine it after every patch update.
If you get the "Server Full" message, you'll probably notice in the bottom left corner of your log in screen that instead of saying "Ephinea v1.0001" or whatever the server name is; it will probably say "Unknown Server" or something to that effect.
(I should have taken screen shots last night. I'm responding via my phone at work.)
So I have to go back into my Windows Defender firewall settings and re-add the already existing PSOBB folder (not file) to the exclusion list.
I've had to do this several times now on my wife's computer (usually after every update) and it'll work like a charm until the next update.
I've had zero issues with it on my computer.
Hope this helps.