Have you access to the server and the logfiles? You could at least check the logfiles of the client (F12 opens the logging window). Does it already connect to the new IP?
Can you connect to one of the demo setups? Demo instances of 9, 10, 11 and 12 now live
Without server access it is hard to guess. Perhaps not only the IP address changed but also something in the configuration? You run NC 12.0.3 before and after the migration?
Can you directly connect via webdav with cyberduck or winscp?