The magnet optos are triggering errors because there is a problem with the 12v supply. Check here for some troubleshooting guidance: http://www.pinwiki.com/wiki/index.php?title=Williams_WPC#Check_Fuses_F115_and_F116_Message

Have you followed the troubleshooting advice I posted earlier? The error on start up sounds like a 12v supply issue which is a very simple circuit on the power driver board and is easy to troubleshoot if you follow the steps on Pinwiki.

Yep! I had a similar issue on a W?D which led me to some issues on the unregulated 12v which were the problem. That's why I'm suggesting it as OP doesn't appear to have gone down this path yet.

He has already tried a new CPU board with no improvement.

That voltage is unregulated so yes it may be higher than expected. 15 volts doesn't seem like it would cause these issues.

C30 could potentially be causing issues. If you have a way to measure capacitance, that would confirm. Otherwise you'll have to eliminate everything else first.

Have you eliminated issues with the J112, J116, J117, J118 connectors themselves? Check the connectors haven't lost their tension. You can rearrange J116-8 as these are keyed the same.

Otherwise, does everything work once you replace the resistors on the opto boards?