Error message:
Error while running event Cursed-FactorUI::on_tick (ID 0)
Cursed-FactorUI/control.lua:420: attempt to index field 'element' (a nil value)
stack traceback:
Cursed-FactorUI/control.lua:420: in function 'init_gui'
Cursed-FactorUI/control.lua:513: in function <Cursed-FactorUI/control.lua:512>

Only other mod is bottleneck.

Fixed? I can't test multiplayer, I've updated FactorUI

If you use the "Factorio Standalone" version and install factorio into multiple sub-folders. You can start up one to host a server on the LAN. Make sure to un-check "verify users". Then with one of your other installs you can now log into the LAN server with a dummy account.