Hi, I've been trying to open the Classy Hats gui, but the button doesn't do anything. I tried to change the key as well with no results. So I found some hats, and notice they say you can shift+use to open the gui as well, but when I did that, the game crashed. Here is the crashlog:

https://pastebin.com/z7iKbyjd

Does this occur in singleplayer or multiplayer? Or both?

I only play singleplayer, I can't help for multi, sorry.

Have you tried redownloading the mod?

Also, did you download it from CurseForge, or another site?

I use the Twitch app, so it downloads for me from Curse. Give me a little time, and I'll test the mod alone and see if there's some kind of conflict with another mod.

Tried it again alone, and received the same issue and same crash.

https://pastebin.com/QUcxmRbx

Update: I've updated Forge to the new recommended build, and the GUI is now working. I guess having a minimum forge version listed would not go amiss in the future.

We weren't aware that the mod didn't work on older Forge versions. It is advised to use the recommended Forge version when present though, and your version was much older than recommended.