Two mods clash inside this pack
Applies to Forge, NeoForge, Fabric, Quilt.
What causes it
A mod patched the game expecting a library or another mod that is missing or the wrong version, so the game crashed when that code ran.
How to fix it
The stack trace names the mod (look for handler$...$modname). Remove or exclude that mod server-side, or ask the pack author to ship its missing dependency.
Usual suspects: the mod named in the mixin or handler frame of the crash.
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