"open_basedir restriction in effect" when editing permalink settings on mapped domain

I set up my first network (subdirectories, not subdomains), and tested domain mapping, using the WPMU plugin. Everything worked fine, until I wanted to edit the permalink settings of the site for the mapped domain: As soon as I click the menu, the setings appear, but on top of the window I get the following message:

Warning: is_writable(): open_basedir restriction in effect. File(/) is not within the allowed path(s): (/srv/www/vhosts/mydomain.com/:disappointed:tmp/) in /srv/www/vhosts/mydomain.com/httpdocs/wp-admin/options-permalink.php on line 130

Despite this warning, I can edit the settings and save them, and the changed settings work fine. Nevertheless, I wonder if I have to worry...

The warning does not appear if I edit settings of a blog that has no domain mapped.

Thanks for advise how to solve this.