The game fails to load with the following mods installed:
JustEnoughIDs-1.0.3-54
betterportals-0.3.7.7
Forgelin-1.8.4
SmoothEntityLight-3.0.6-MC1.12.2

Removing any one of these (or better portals and forgelin as a pair), causes the game to load just fine.

Here is the error log: https://pastebin.com/g33p0i0a

I'm putting an issue on each mods tracker since they only seem to have issues in tandem.

Here are the issues:
Johni0702/BetterPortals#493
DimensionalDevelopment/JustEnoughIDs#125
#12

SEL's class transformer is actively loading game classes which isn't allowed and can under certain conditions (hence why it stops happening when one of them is removed) trip up the Mixin library which both, JEID and BP, are using.
This looks like the same issue with SEL + VanillaFix (which uses Mixin) + Optifine: #6

Based on the crash report, it's the SELClassWriter which is doing that. Looking at its usage (ref), I think this can be easily solved by just not re-computing the frames (instead keeping the ones from the class reader). The SELClassWriter can then also be removed.