NeoForge 1.21.1 ver: 21.1.117
Look at how the error occurs, the mods I used were in there. Had to press ctrl + alt + r to restore to normal, when the error occurred, the buttons above did not show anything when pressed.

I am like 99% sure this is not FM's fault. It's probably caused by the mod config screen of that other mod.
You should report this to the other dev, except it happens with every mod config screen.

Will close this for now, but if it happens with every mod config screen, I will open it again.