I had the same problem as most people where my images uploaded fine but no thumbnails even though it appeared in the thumbnails loading bar that it was working.
Creating new thumbnails in the manage gallery page didn’t work at first either.
Now, I’m not sure if it was a complete fluke or not but on the next gen options page in the Thumbnail settings tab, I changed the quality from 100% to 95% and lo and behold, the next time I created new thumbnails, it worked.
My slight reservation is that I’ve been trying and changing so many things that I might have done something else that I can’t even recall!
Anyway, good luck all.