You can join a server no problem if you relaunch a modpack. But if you happen to relog, with the mod in your client it will throw null point errors and make you have to relaunch to rejoin.

After removing the mod from the client me and a few other people no longer had issues connecting to a server.

Connecting Log to a server
https://gist.github.com/litjohn50/257b40a792bc383c2c06c295d1deeb03

Subscribed, I can confirm this issue.

Also seeing this in my modpack.

fixed in 3.10.0