Warnings printed when trying out the 'send to friend' function

I just tried out the new 'send to friend' function. For every email address I entered, I got the warnings after submission:

# warning: _print_var_generator(): open_basedir restriction in effect. File(/sites/all/modules/print/print.css) is not within the allowed path(s): (/var/www/vhosts/sg-kaarst-volleyball.de/subdomains/cms/httpdocs:/tmp) in /var/www/vhosts/example.com/httpdocs/sites/all/modules/print/print.pages.inc on line 163.
# warning: _print_var_generator(/sites/all/modules/print/print.css): failed to open stream: Operation not permitted in /var/www/vhosts/example.com/httpdocs/sites/all/modules/print/print.pages.inc on line 163.

and this one at last:

# warning: _print_var_generator(): Failed opening '/sites/all/modules/print/print.css' for inclusion (include_path='.:') in /var/www/vhosts/example.com/httpdocs/sites/all/modules/print/print.pages.inc on line 163.

However, the mails were sent.