One of my friends can't join

I have played on my modded Aternos server with my friends for a while now. Suddenly one of my friends can't join, but everyone else can. When he tries to join it says that he lost connection and in the server log it says "could not be synced because server networking isn't set up yet". I added a mod yesterday but thats not the problem, since I tried to remove it and it still said the same thing. He has downloaded all necesary mods and I tried to use the DYN IP. What do I do?

The mods that we have are: Origins, Mob origins, goat origin, extra origins, collection origins 1 and 2, medieval origins, Pehkui, and fabric api