With Forge 35.1.36, on Minecraft 1.16.4, and Supplementaries 0.9.20c, I get a crash when switching texture packs. Full crash log here.

Looks like the issue is in this line:

else if(ServerConfigs.cached.THROWABLE_BRICKS && (i.isIn(Tags.Items.INGOTS_BRICK)||i.isIn(Tags.Items.INGOTS_NETHER_BRICK))){
    event.getToolTip().add(new TranslationTextComponent("message.supplementaries.throwable_brick").mergeStyle(TextFormatting.GRAY));
}

I'm not experienced with either java or forge, but this might need to either wait to register this event until the tags are fully bound, or just do a simple check to see if the tag is ready before adding this?

yeah Just added a null check there. I've got another report that said the same

I have to say that I still can't replicate it. I've posted a beta version on my discord so you can tell me if it's still not working

Issue seems to be fixed with 0.9.21, thanks!