Adjust the carriage position for light detector back and forth a few mm in 0.5mm steps. Remember to reset diodes every time.
Monitor the voltages while you perform calibration and see if you notice a sudden drop. If you notice a drop, start disconnecting the LDD's.
You said you replaced DDB and LD - were these new or known to be in working condition?
Logs may be very useful - check the timestamp in the error log then look in the general log to see what happened before it. You can PM me with the logs attached