Issue is detailed here: CalaMariGold/TREPIDATION#4

I'm not sure if this is on VanillaFix's side or not, but I figured I would report it here. Let me know if you need any other information. Thanks!

I checked the log during this issue and got this

[16:25:03] [Client thread/ERROR]: ########## GL ERROR ##########
[16:25:03] [Client thread/ERROR]: @ VanillaFix GUI Loop
[16:25:03] [Client thread/ERROR]: 1280: Invalid enum
[16:25:10] [Client thread/INFO]: [FANCYMENU] Updating animation sizes..

VanillaFix always had problems with FancyMenu, that's why it's listed as incompatible with FancyMenu:
http://fm.keksuccino.com/en/incompatibility-list

FancyMenu v3 is now out and it's a full rewrite, so I'm closing this for now. If it still happens, please open a new issue, thank you.