Versions

Betweenlands version:

TheBetweenlands-3.3.10-universal

Forge version:

forge-1.12.2-14.23.4.2726-universal

Singleplayer or Multiplayer:

Multiplayer

Installed mods:

https://minecraft.curseforge.com/projects/ancient-towncraft/

Description of the problem

How to reproduce:

After each update the old core mod is still in the folder and getting loaded.

This was discovered when searching this crash to find the cause, not saying it is related, but I hope it is.

https://gist.github.com/ProsperCraft/e7f92dce039880b4701e877d9398f20f

The coremod thing is just what Forge's best practices with coremods recommends, the coremod should be in a seperate jar. Hopefully the new mod dependency system that's supposedly coming with Forge 1.13 will fix this problem and take care of outdated coremods. Meanwhile I'll try adding a check that makes outdated coremods not do anything.
Regarding the crash: It doesn't really look like something caused by the Betweenlands. Is there a specific reason why you think it might be the Betweenlands? Have you tried without the Betweenlands installed, etc.?

I don't suspect The Betweenlands specifically, I just found the multiple coremods when reading that crash report.

Other mods delete their old core mods, perhaps you could have each version do that for The Betweenlands as well?

Looks like Forge now allows specifying a maven version in the coremod, maybe that could fix it.

I may be wrong, but I think Immersive engineering removes their old core mods.

Ah yeah, IE is using the maven version attribute which fixes this problem:

• Added support for Forge's memory Maven repo, this makes deleting old coremod versions unnecessary in Forge 2656+ (latest) (Malte)

Other mods delete their old core mods, perhaps you could have each version do that for The Betweenlands as well?

Which ones for example? I don't see how the coremods could delete themselves before or after being loaded.