CliveS

This is becoming a common problem. Panel voltage is so high these days that 3S of 72-cell panels (they are nearly all 72-cell again these days, especially the panels that are over about 315 W) is too much voltage. Yes, 121 V is within the specifications, but it seems to cause problems. Another forumite found that the SCC gets much hotter when running 3S, so it may be shutting down due to over-temperature. The Synapse is basically an Axpert copy or work-alike; they may not have copied the cooling system properly.
Rewiring to 2S6P is a pain, but it will likely fix your problem.

Thanks Guys.  I've got them back here later this week to fix this issue.