Were any crash dumps created?
Do other FTP sites still work OK in Opus?
What happens if you connect to that same site using other software on the same machine? If the crash is happening due to security/firewall software intercepting that particular site, then you'd need to test on the same machine to find out. (Although it's also possible the same software may treat different programs differently.)
Please send us the crash dumps if there are any, as they may tell us where the problem is, and whether it looks like Opus or something else is causing it.