A client-only mod is in the server files
Applies to Forge, NeoForge, Fabric, Quilt.
What causes it
A mod that only works in the game client was installed on the dedicated server, and it crashed trying to load client-side code.
How to fix it
Exclude the mod from the server (Mods tab, Excluded from pack). Players keep it in their client pack.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log