This looks to be a near-identical crash report to closed report #825. Primary error seems to be:

net.minecraftforge.fml.common.LoaderExceptionModCrash: Caught exception from The Betweenlands (thebetweenlands) Caused by: java.lang.IllegalAccessError at net.minecraft.util.math.BlockPos$MutableBlockPos.<init>(SourceFile:276) at net.minecraft.util.math.BlockPos$MutableBlockPos.<init>(SourceFile:267) at thebetweenlands.common.world.gen.feature.tree.WorldGenRubberTree.<init>(WorldGenRubberTree.java:18) at thebetweenlands.common.registries.BlockRegistry.<clinit>(BlockRegistry.java:305) at thebetweenlands.common.registries.Registries.preInit(Registries.java:11) at thebetweenlands.common.TheBetweenlands.preInit(TheBetweenlands.java:126)

However, while I thought it might be an issue with Dynamic trees, #825 did not seem to have it. Searching has not resulted in a fix, though I did implement the ProjectRed update as indicated in the other active crash reports. No resolution.

Please let me know if there is anything else I can do to help resolve.

Versions

Betweenlands version: 3.4.9

Forge version:14.23.5.2825

Singleplayer or Multiplayer:

Both

Installed mods:

https://paste.dimdev.org/oxetobuleg.rb

Link to full crash log

https://paste.dimdev.org/ipefekizex.rb

Description of the problem

How to reproduce:

1. Install listed mods
2. Run game, either on client launcher or server.
3. ...
4. No profits.

Expected result:

Client or Server Initialization

Actual result:

Crash

Pictures:

It's an issue with FoamFix, follow the fix here asiekierka/FoamFix#200

You are my hero. Thank you. Hopefully the next person who runs into this will get this page on google. Changing the config\foamfix.cfg file to read this way solved the problem:
coremod { B:forceDisable=true }

This was NOT a Betweenlands issue. Sorry for the false alarm.

As I look at this, I'm starting to wonder if it's a Biomes a Plenty issue... Doing some more testing.

Nope, that's not it. If I remove it, I get worldgen issues from somewhere else. I'm hoping that if I/we can track this back, it will help someone else.