The unit will work fine in direct sun, so I don't think that's the "cause" but something else could be going on that is affected by the sun. As it's a used unit, you don't know what the previous owner did or didn't do.
It doesn't help for the weekend, but you could move it to the shade (just as a double check) and try that. On Tuesday, call CS and they'll help you out. Could be something simple, or maybe not. Sorry I can't help troubleshoot it.