Hey lucas_on_votec,
From this error message it seems like you've put the -sources.jar file or one of the old -dev.jar files in your mods folder. These versions are meant for development and cannot be launched in a normal environment.
To fix this, make sure you're using the normal jar file without any "-sources" or "-dev" at the end. This should allow you to launch the game without any issues.
Hope this helps!
