Forge Server Tools · every failure mode · guides
Players are being kicked for a mod mismatch — ERROR
The client's mod set doesn't match the server's for mods that register a network channel. Any version difference on a networked mod is enough.
The listed mods must be the SAME VERSION on both sides. If you updated a mod on the server, everyone needs the same jar. A running client keeps the jar it launched with, so players must fully restart the game, not just reconnect.
Search your crash report or latest.log for any of these:
Mod rejections Incompatible mod set Connection closed - mismatched mod channel list
The free edition covers the six most common startup failures, and this is not one of them — it is in the paid tool, which checks all 24 and names the mod jar from the stack trace. The fix above is the same one it prints, so you do not need it to solve this.