Hi,
I had the same issue, for me it seemed to be the "open_basedir" restriction, even if I had the Php /tmp/ dir writeable and in my Php.ini for the specific site,
Perhaps not the solution, but when I set "open_basedir none" it works, although I really do not like this setting from a Security Point of View....
But it is a step forward perhaps, in troubleshooting ?
Edit:
When setting the open_base paths back and have the correct dirs setup and permissions, the 500 Internal Error comes back, when trying to print..