Answer lies here:
https://devforums.apple.com/message/922989#922989
I found the reason. In a viewController, which was not yet allocated, but included in the app, there was following implemented (in the .m-file above implementation viewController):
@implementation UIImageView (UIScrollView)
- (void)setAlpha:(float)alpha {
..........(no difference if here is some code or not) ...............
[super setAlpha:alpha];
}
@end
Putting above in comment solved the problem, even though the button which didn't display the image wasn't in a scrollview.
与恶龙缠斗过久,自身亦成为恶龙;凝视深渊过久,深渊将回以凝视…