I don't see why I need to write a bug report if I don't even know what happened but I have a crash report which should tell you why The Betweenlands is causing my game to crash. https://paste.dimdev.org/aqohulixop.mccrash

looks like you have more than 4096 items in your modpack, which w/o one of the mods that increase item limit will make your pack not work.

just Enough ID's is a commonly used mod for the fix, tho there are others.

^
Yep, that's it.