What does a white diagonal line mean on an Airbus ECAM DU?
Display Management Computer issue:


  If a DMC fails, all information is removed from the displays it
  controls and replaced by a white diagonal line to indicate a working
  display with a failed source. ECAM will detect the failure and provide
  a sequence of alerts.
  
  Due to the priority rules, the E/WD display will
  move to the lower DU if it was DMC1 that failed. The EIS DMC switch is
  used to replace the failed DMC with DMC3.


Source



Steps to clear warning:


  First select DMC #3 to isolate fault to DU or DMC. If DMC is at fault
  reset above associated circuit breaker for 5 seconds. If fault is with
  DU reset these (CAPT PFD/ND = G3/E1) (FO's PFD/ND = R3/R4) (Upper ECAM
  = E5) (Lower ECAM = R11).


Source



More in the maintenance manual at AMM31-60-00-710-001. (EIS start).