I have solved the problem. The issues had nothing to do with my code. Just in case it helps anyone else: In the file yourappname.apparmor (which is automatically generated by clickable) you need to add the policy group "picture_files_read". I assumed "content_exchange" would suffice but it doesn't. So this issue is solved

Thanks so much @lduboeuf for pointing this out!!! This certainly explains all of the above described behaviour!!!