It's the background.
It won't change anymore. I have the settings for it, changing every 30 min, already for 5-6 weeks and it worked well until today.
May it be because I added some more pictures to the those I have and it now exceeds some kind of a limit or something?
For future reference: the only reason I can see this happening is power saving or any such power profile being enabled. If this is a laptop, that was most likely the case. Also @Ravallo @Doc @iverson954360