Warning messages when trying to access "Pro Sites Upgrade"

Hi Support,

I am getting warning messages (see below further information) when trying to acces the Pro Sites upgrade page.

Further information:
- URL: http://sitepr.es
- Besides the "WPMUDEV Dashboard" Pro Sites in the only plugin installed - so this can not be due to a plugin conflict...
- The only change made in "Pro Sites" is in "Settings" where "Rebrand Pro Sites" has been changed to "PLUS Sites" and "Admin Menu Button Labels" has been changed to "PLUS Upgrade"
- The first time I encountered the error, was after:
1. I made a sandbox environment for the setup, URL: http://sandboxpr.es with just WPMUDEV Dashboard and Pro Sites installed/activated (at the moment there is no installation at this url due to trial/error for sitepr.es)
2. I then made the same installation at sitepr.es where the warning messages then occured.
- I have tried using another theme (Twenty Eleven) with no results
- I have tried to uninstall/reinstall plugin with no results
- I have tried to uninstall/reinstall the WordPress MU installation with no results.

Look forward to your support,
Best Regards
Jesper

The warning messages are as follows:

WARNING MESSAGES IN "HEADER AREA":

Warning: session_start() [function.session-start]: open(/tmp/sess_a2cf4d69513a504816f7024c0ded7b6c, O_RDWR) failed: Permission denied (13) in /home/sitepr/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 698

Warning: session_start() [function.session-start]: Cannot send session cache limiter - headers already sent (output started at /home/sitepr/public_html/wp-content/plugins/pro-sites/pro-sites.php:698) in /home/sitepr/public_html/wp-content/plugins/pro-sites/pro-sites.php on line 698

WARNING MESSAGES IN "FOOTER AREA":

Warning: Unknown: open(/tmp/sess_a2cf4d69513a504816f7024c0ded7b6c, O_RDWR) failed: Permission denied (13) in Unknown on line 0

Warning: Unknown: Failed to write session data (files). Please verify that the current setting of session.save_path is correct (/tmp) in Unknown on line 0