Electric Water Heater Bugs

[RLB01]

i installed a Rheem Rtex18 electric on demand hot water heater. works fine with sink, but when we installed the tub the heater errors out when we turn the tub faucet on. the only diff in plumbing is the tub has a Watt mixing valve and the sink does not. dont see why that would matter. the error is "E5" dont know what that means. online resources say that error means combustion chamber or venting is dirty, but this is electric. HELP!!!

 

[Fitter30]

E5 blocked or improper venting heres the manual page 26
https://www.tanklessking.com/mwdownloads/download/link/id/467/