Optifabric is pretty important as it's the only way to get optifine with fabric
I checked if it wasn't the mod itself and it loaded (deleted it)
Checked it without optifabric and it loaded

which version?

In my identical situation there are Minecraft 1.16.4, ImmersivePortals 0.49, OptiFabric 1.8.10, Optifine 1.16.4_HD_U_G5 and Fabric API 0.26.1 respectively.
Game just crashes in about a second after getting in the world.

@Lyapsus Did you disable render regions?

Oh sorry forgot about it. But I dunno where to find this option, gonna try it right now.

@Lyapsus Video options - performance (maybe)

@qouteall sorry for so long.
Well, now it loads in the world at least:) Gonna poke around lil more on a week to test things further.
Thanks a lot:)

@qouteall uh oh, the new crash.
I tried to take screenshot of open portal with 4x resolution from optifine settings and all I've got is 0 byte screenshot file and these logs.
OptiFabric 4x screenshot crash.txt
Gonna try other options like 1x screenshot size...
What do u think about compatibilty render mode in IP configs tho? It was turned on. What does it do?

edit: Well i can say that it could be vram problem or smth because without shaders 4x screenshots work perfectly fine.
With shaders portals are lagging as hell being in sight also.