When uploading an image that falls within allowed dimensions it is rejected if the EXIF dimensions are set.
Tested with attached 120px*120px image on phpbb.com.
"The submitted avatar is 5312 pixels wide and 2988 pixels high. Avatars must be at least 20 pixels wide and 20 pixels high, but no larger than 120 pixels wide and 120 pixels high."
Those dimensions are in the EXIF data, this can occur if the software you are using to edit an image does not change the EXIF information.