Without more Information it could be a pletora of problems:
1 Mod conflict - Just go for Brita and then add mods till the problem is back
2 Missing dependecies - Brita is basically 3 mods Armor/Weapons + the "Mainmod" - last one missing maybe?
3 Snakes Modpack if used wont work with Brita
4 servertest.ini problems - if not set up correct it can lead to a shitload of trouble, make sure every mod and id is done the same. Meaning Brita weapon mod name and id first, then armor and so on. Dont mix up here.
5 Anythign else i cant know without seeing the serverlogs and/or the setting files