Ran into the same problem earlier this week. I'm pretty sure this is not your issue, but then again...
I had been testing my sequences with XL and had the Light output button on; I wasn't playing any sequences, just Idle.
But I was also running my FPP to test it's functionally; running a test sequence. What I saw was what you see. Flashing/shimmering lights when it should be solid. The reason this happens is not clear to me, but I think it has to do with a conflict between XL output and the Pi trying to run a sequence. Possibly the network traffic from both were sending different commands to the controllers.
All be told, as soon as I turned off the XL lights button, all was good.