I'm experiencing the identical behavior with my power supply and was happy to stumble on this thread. Bringing the PSU up slowly, I was happy to see the (unloaded) voltages within specs, but it dropped out to 1V when connected to the motherboard. The 12 and -12V lines are fine.

At first I thought the motherboard was shorted, too. But I measured 90 ohms across the 5V-GND, which is close to the manual spec of 100 ohms (Sol systems manual, p. III-11).

Looking forward to seeing what is going on with Q1 and Q2 on the regulator board.