mod has been loaded on the wrong server side

What this actually means

A mod that only exists for rendering/UI was placed in the server's mods folder. Common culprits: shader loaders, minimaps, HUD mods, texture utilities, animation mods.

How to fix it

Remove the named mod from the server's mods/ folder. Clients keep it. If players then get kicked for a mod mismatch, mark it client-side in your launcher/pack config rather than putting it back on the server.

What it looks like in your log

Search your crash report or latest.log for any of these:

has been loaded on the
is client
net.minecraftforge.fml.ModLoadingException  ...  client

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.