Cycles - Image/Environment Sequences on some Img Seq only Black

I’m trying to add a 360° Video as a Environment Background. So i’ve tried the Video File in “Environment Texture” gives only Black. So i converted the Video to .jpg sequence - only Black. So i’ve converted it to .png - only Black.

It works on some other image sequences, but on others it’s Black as well. (Same with "Imgae Texture)

I could not find any Pattern or anything why some img seq. works and others not…

(I know i have to set Frames and tried Auto Refresh - doesn’t do anything…)

That’s pretty annoying… I know i can make it work with python - It’s more - i wanted to make a Bug Track, but what should i write? It works sometimes and sometimes not? hmm.

Anyone else some hints or similar issues so we can tackle this down?