And after reinstalling the program again, I realize now I don't read instructions. It clearly states the following for the Domain tool tip:
Hostname or IP Address of this server.
So when I changed the Domain, it broke the connection to the server somehow.
While I set the correct IP address in the XMPP settings, which allowed me to sign in correctly, sending messages didn't work, and I figure this was the issue as changing it back to the server's host name fixed the issue.
What a weird problem to have...