The pack’s custom menus are missing their own files
Applies to the game client.
What it looks like in your log
[FANCYMENU] Failed to read texture from file
What causes it
FancyMenu is rebuilding the game’s menus from a layout that points at textures, sounds or backgrounds which are not in this instance. A half-loaded layout still swaps widgets in and out of the screen, which is how menu customisation turns into a crash on click.
How to fix it
Reinstall or repair the pack so the missing files come back. If menus keep crashing, remove FancyMenu: it is client-side only and costs nothing but the custom menu art.
Not sure this is your problem? Paste your log and Roastd will tell you which of these it actually matched, and what else it found.
Check your own log