I logged into a friends modded multiplayer server. I was the only one online. I walked around for a couple seconds before crashing while eating a piece of bread.

Crash report can be found here

The full list of mods can be found on the crash report
I believe the relevant mods are:
Quark r2.4-316
Optifine_1.16.5-2HD_U_G7
QuarkOddities-1.16.3

This issue appears to be an issue while trying to render a skeleton's armor layer, and an array out-of-bounds error. There is also a NullPointerException on the skeleton's "vehicle" tag but that could be normal

The game is in hardmode right now, so mobs can spawn with armor normally

I found the location in world where the error log says the skeleton was (image here), and there does not to appear to be any mob spawner nearby, just regular cave

I was able to re-log on without experiencing the error again

From what I can tell this issue is different from Issue 3321

I can't really give you much help regarding a render issue with optifine.