Worked for me too.
For anyone else wondering what exactly the problem/fix is:
This mod here:
Extended Weapon Customization at Warhammer 40,000: Darktide Nexus - Mods and community
got an update
and now requires this mod here:
MasterItems Community Patch at Warhammer 40,000: Darktide Nexus - Mods and community
If you run weapon customization without the newly required mod, you will get backend errors.