Thank you very much, Erel.
There was indeed a problem with case sensitivity as the file was with a lower case "p" ind the files folder, but when copied to the shared folder it appeared with a capital "P". This, however only solved half the problem.
In my app the user can choose a file anywhere on his HD, which is then copied to the Files folder, but in release mode, the file is physically copied to the Files folder, but still is not recognized by the app, whereas this works perfectly in debug mode.
Any idea why?