I suspect something in your image sequence is problematic like the way files are named. The files should be named with the same number of numeric digits using leading zeros. For example, Milkyway-001.jpg … Milkyway-999.jpg and NOT Milkyway-1.jpg … Milkyway-999.jpg. I confirmed that it used to load without leading zeros in an older version but no longer in 19.06.15. I have not figured out why because there were no changes in the obvious place to look, but what I have stated is the better and more expected way to make an image sequence. I will keep looking, but that advice is all I can offer for now.