Version Information

Minecraft: 1.12.2
Forge: forge-1.12.2-14.23.5.2860
Iron Chests: ironchest-1.12.2-7.0.72.847

Report Information

Chests from ironchest can be placed in the overworld and used by players that don't have optifine installed, but when someone with optifine installed tries to open a chest it crashes. Crash happens in singleplayer and in multiplayer.
We tested it with the iron chests and gold chests but not any others.
Optifine filename / version is optifine_1.12.2_hd_u_c6, if it helps. We didn't test it with other versions of ironchest/forge/optifine so it's possible it's just an issue with this combination specifically. Not sure though
Uninstalling optifine fixed the issue completely, so I think it's just optifine, but this is a screenshot of all of the mods we're using.
None of us mind not using optifine so it's not a huge loss.

Steps to Reproduce

Have optifine installed and open chests from the mod (at least iron and gold, maybe not others?)

Crash Log

Forge Log

As far as I know, it's generally not recommended to use ancient OptiFine versions with modern Forge versions. OptiFine has to reimplement chunks of Forge within itself, and since the older version reimplements an older version of Forge, this can cause issues. The version of OptiFine you're using dates back to 2017. Does the issue occur with OptiFine F4 or newer?