Today I tried to enter the Nether for the first time in my save. I built the portal, lit it, and walked in. As soon as I did, it briefly showed me terrain before crashing the game with an ArrayIndexOutOfBounds Exception. I've tried rebuilding the portal and entering it in a few different locations, but the result is the same.

Interestingly enough, if I restore from a backup and use the /execute command to teleport myself into the Nether it loaded it just fine. The second I brought myself back to the Overworld and built a portal and entered the real way, crashed once again. Only details I can gather from the Nether side is that the portal is getting generated inside of or on top of a Nether Fortress. If that helps at all.

I've attached my crash log, though I'm not sure if it'll be that helpful.

crash-2021-07-31_17.39.46-client.txt

Once i got the same error but with the number -86

judging by the logs this seems to be an issue with sodium

I encountered this issue today.
When I made the first nether portal by lighting up the obsidian with Flint and Steel, minecraft crashes immediately.
A quick fix of my problem is to remove "magnesium-1.1.2.jar" from mod folder.

Although my Minecraft Crash Report point the crash is related to Sodium, Sodium seem not available on the Minecraft 1.16.5's Forge. Therefore, this mod pack uses Magnesium as a replacement of Sodium. As a result, maybe disabling Magnesium is also a quick fix for your issue too.