Using MC 1.11.2, Forge 13.20.0.2312, preview-OptiFine_1.11.2_HD_U_C1_pre and WearableBackpacks-1.11.2-2.4.1.

Crashlog: https://gist.github.com/vpontin/cc9a9d0cb70019a96c83db0f46fa8c3b

Dropping issue here due to the various net.mcft.copy.backpacks.*** in the stacktrace.

Can you test if this issue still occurs without OptiFine?

Dropped the optifine jar and the crash's gone. Sorry for waste your time.
Mental note: i really need to find an alternative, at least for better chunk loading and shaders

For the record I don't mind if you keep this issue open until it's properly fixed. Will you contact the OptiFine team for them to have a look at it?

Unfortunately I did not write the model loading code and don't understand it well at all. @asiekierka, if you don't mind, could I ask you for your expert opinion on this matter?

@copygirl Looks fine to me, especially as I use similar logic in Charset.

@sp614x ?

Oh sorry, i always close issues when is determined this is related to Optifine, because several modders says "I can't cope with that".

But if your wants, i will reopen.

@vpontin Closing it doesn't mean the issue will disappear. Let's wait for OptiFine devs to weigh in first.

I don't see issues as something bad. It's a reminder to myself and information to other users, maybe even developers, that a problem has been found and is present. If you were to close this, another user might be more likely to report the same issue. And I like to keep them around even if another mod is at fault, since it still affects users of mine. If you look at issue #3, it's still open even though it's marked as "wontfix".

copygirl - great practice. Found this useful as I noticed this issue myself. @vpontin did you ever get a response from Optifine?

The relevant issue is open on the OptiFine tracker, so this can be closed I suppose.

Sorry to not report back - the fact is i ditched from optifine months ago - i became aware of the bizarre ways such closed source mod uses to "deliver performance" - not worth it since a lot of mods break ( another to be cited beyond WB is ComputerCraft; monitors simply dont work with optifine installed).

Sorry to bother asiekierka in first place, Optifine jar should be in my trash long before.

Out of curiosity I checked how Wearable Backpacks works with OptiFine + shaders.

Tested Forge #2588 + OptiFine E1 for 1.11.2 + WearableBackpacks-1.11.2-2.5.0.
Loading and unloading shaders is OK, no crashes.

Also tested Forge 1.12.2 and latest OptiFine + WearableBackpacks for 1.12.2.
Everything is OK, no crashes when enabling/disabling shaders.

ComputerCraft was fixed to work correctly with shaders: sp614x/optifine#1237 (comment)
It uses custom rendering that was not compatible with the shaders shadow map.