Best guess, your image is in indexed mode with limited colours.
example: https://i.imgur.com/g7EcCw5.mp4
edit: reading the post again
or maybe not: Can you share one of the problem images.
example: https://i.imgur.com/g7EcCw5.mp4
edit: reading the post again
Quote:Usually when I open the tool for changing levels on a picture nothing happens until I press ok.
Lately as soon as I open the tool the picture turns an ugly blue and will not do anything else.
or maybe not: Can you share one of the problem images.