Hello Davo,
I just let you know that I have upgraded to Moodle 2.2.11 (the latest version supported of Moode 2.2) and the problem also persists. Since only 'Upload a single file' is affected there is no point in wasting more time, as we can use a different Moodle assignment.
Moodle 2.3 incoporates a new type of assignment any, so the planning is to migrate but to higher versions of Moodle, instead of Moodle 2.2.11
Thank you very much for your help.