It proved that there was some hick-up with my Osclass installation. All worked fine except the image upload.
I tried to create a new database to see if it was database related but it was not.
Finally I took the following steps:
Create a new database
Installed Osclass via my provider on a new subdomain with the new database.
Uploaded the database content of the old database in the new one
Delete directories oc-admin, oc-include, oc-content
Uploaded directories oc-admin, oc-include, oc-content from my saved domain.
This made it all work.
Thanks for your help!