Right after a color DMD was installed, the game reported several switch errors at once, some buttons stopped working and mechanisms misbehaved, although the solenoids fired in test.

Many switch errors and odd behaviour after fitting a color DMD: check grounds

Cause: A loose ground: the ground straps and board mounting screws in the backbox were not tight after the boards and display had been worked on.

Checks

• Confirm the 12 V supply is present, since the switch matrix and the color DMD both use it.
• Check every backbox ground strap, the board mounting screws and the ground wires on the CPU board connectors for looseness or wires pulled out of their pins.

Fix

• Tighten all ground straps and board fasteners and reseat or repin any loose ground wire, then reboot and recheck the switch test.

Systems

• WPC Dot Matrix
• WPC Fliptronics
• WPC DCS
• WPC-Security (WPC-S)
• WPC-95

Related tips

• Row or column "ground short" errors
• After a new U20, work every flagged switch once
• Switch rows 1-3 show shorted to ground: opto board missing its column strobe
• One switch also triggers another: corroded switch connector on the CPU board
• "Ground Short Row" message
• A whole switch row or column is dead
• A dedicated (direct) switch does not work
• Using the switch edge test

Source: Pinball Repair help group!! (Facebook group)