A certain content-type in my installation does not save images in it's gallery anymore.
The image is being uploaded, registered in file_managed with status 0, and stored in sites/default/files.
On save the status does not get updated to 1 in the database, therefore the file gets deleted.
I entered some debug code, and it turns out that file_field_presave() never even gets called for the uploaded file.
Curiously on my testing copy of the same system it works, the function gets called and the status is updated to 1 in the database.
I compared the content-type and field-type settings, and they are the same.
I also compared all drupal-files again (diff -r), except for database configuration ant .htaccess they are the same.
update.php doesn't do any update, and the modules have no security updates pending.
How could I debug the problem? I don't know how to look deeper. Is it possible that the database is somehow inconsintent?
Thanks a bunch for your help people.