hey, i have like the same problem, but at the same time completly different, my friend got a server with FTB Trident, and it worked one day. when he wanted to start it today, we first got the message, that ther server isn'T on, then he restarted, jet we can see it online, but if we wan't to join we get the message:
This server has mods that require FML/Forge to be installed on the client. contact your server admin for more details