I checked & resetted the permissions for data and uploads directory 3 times. It is definiley correct. I also checked permissions for images directory and all php and sql files etc. They are all correct as described in the installation instructions.
I tested now with a third photopsot 531 installation. Again on the same server. This one uses the info in the admin options jpegtran field. THis works fine too.
So it is only one of 3 installations that is not working properly. Something must be different in this specific installation. I only do not know what. I tested now everythind as admin, to be sure that it is not a userright problem.
This third installation is a french language installation by the way. So it seems not to be a language problem.
If you want to, I can give you access to the installation and the server.