Forge version: forge-1.12.2-14.23.5.2805
Twilight Forest version: twilightforest-1.12.2-3.8.689-universal
Link to crash log: link

What I expected to happen:
Twilight Forest loading normally with all it has in it for my friend who got on the server earlier than I did
What happened instead:
the server crashed

A similar issue happened too when i was using Primitive Mobs, do you have any idea what is causing this?

Duplicate of #660. The issue is caused by the Spiders trying to access a normally private class (which is made public by Forge), but OpenJ9 seems to prevent the class transforming.

Would changing from OpenJ9 help? Or would it cause more issues somehow? I'd really like for my server to actually work

Yeah, if you can change it from OpenJ9, it should resolve this issue.

Oracle JRE is preferred as much as possible. Depending on the server and what kind of access, you might need to manually specify that you want to use non-free binaries.