This is probably due to a worn-out measuring card. The measuring card holds the voltage measurements and this circuit tends to last between 8-10 years depending on the ambient temperature. See table below:

To verify if the voltage circuit is failing on its measurements is by measuring directly on the voltage-measuring terminals of the ML-2 unit. Terminals 79, 81, 83 is the generator voltage and terminals 85, 87, 89 is for the busbar. 

Write down the measurements and compare them to all phase-phase measurements on display. These can be fund by moving the display-cursor under V1 and then scroll up or downwards till measuring's are found.

Should these deviate from each other the measuring-circuit is bad. 

From here call or write DEIF support for further help.