Have you considered taking a picture maybe sharing your files?
A picture of the preview? I attached a screenshot....you might be able to make out Christmas candles that go out the top of the tree and scroll out the top of the tree and reenter at the bottom of the tree.
As to sharing files I could do that if I knew what files would help.
I suspect that this has something to do with the setup and preview numbers not matching.
I exported an XL sequence to HLS just to check to see if it was just my XL preview and the HLS preview doesn't work either.....so it must be a channel assignment mismatch of some sort...
UPDATE: I found it. When I updated to version 38 somehow one of my universes lost some channels in setup.... It should have had 510 but was only sitting with 300.... I was thinking of renumbering things to play around to see if I could figure out where the discrepancy was. I was copying my channel assignments into a spreadsheet when I noticed the incorrect number. Never had that happen before on an upgrade. But all is well now and working as usual.