Thank you @Gabe
Meanwhile, I discovered that the problem occur when an image has a certain color CMYK profile assigned and i am using buit-in presets.
It works well if the image doesn't have assigned any specific CMYK profile, or I choose. But if I want to convert it in RGB, I have the error.
If I choose CMYK color space, it works. I also noticed therea are problems with AfPhoto in opening some tiff files made in photoshop. I have to open them in Photoshop, uncheck any color profile, then I am able to open them in Affinity. Can you address this issue? my bests,