The display board is not receiving commands from the CPU over the 26-pin ribbon cable.
The splash screen that you're seeing at boot time is the display processor's boot screen. After that the display processor will idle and wait for commands over the 26-pin ribbon cable. When a command is received, the display processor responds by sending an animation to the DMD.
If you've tried swapping the display controller and ribbon cable, the problem may be with damage to the data output buffer (U201) on the CPU board.