Two Curseforge users (davdthethird, vhslance) reported a Minecraft startup crash when combined with Immersive Portals. Configuration:

minecraft 1.15.2 and forge 31.1.0
immersive-portals-0.4(for+mc+1.15+with+forge)
MixinBootstrap-1.0.x(tested with 1.0.0 / 1.0.1 / 1.0.2)
water-erosion-1.15.2-forge-1.0.1

--
 ERROR
 Game crashed
 An unexpected issue occurred and the game has crashed. We're sorry for the inconvenience.
--

Definitely unexpected. I don't see how these mods could be interacting. Initial guess is the SDK translation interface layer.

Running in dev the logged Fatal error:

[main/FATAL] [mixin/]: Mixin apply
failedassets/immersive_portals/immersive_portals.mixins.json:MixinEntity -> 
net.minecraft.entity.Entity: org.spongepowered.asm.mixin.transformer.throwables.InvalidMixinException
@Shadow method func_174813_aQ in
assets/immersive_portals/immersive_portals.mixins.json:MixinEntity was not 
located in the target class net.minecraft.entity.Entity. Using refmap main.refmap.json

The MCPBot CSV says func_174813_aQ is getBoundingBox() which is a valid function on Entity.

Hello, I was made aware of your mod by this issue LXGaming/MixinBootstrap#5

This issue here is caused by your mod water-erosion and immersive-portals having the same refmap name main.refmap.json, whats happening is your refmap is getting overwritten by immersive-portals which is then obviously causing major issues.

The fix for this is to simply rename your refmap so that it contains your mod id. Example: erosion.refmap.json, this needs to be changed in your build.gradle and the refmap entry in erosion.mixins.json

Ah ha! Thanks! This is my first Minecraft mod, so I didn't realize my refmap would end up in the same place as others. I was testing this as the solution last night. I'll release a new version right now.

Fixed in 1.0.2 and 1.1.0-alpha2.