Drupal 7 Warning

Hey there!

At first, sorry for my bad enlish - I try my best :)

Since today, I get this Warning Message:

Warning: file_exists(): open_basedir restriction in effect. File(/themes/default/default.css) is not within the allowed path(s): (C:/Inetpub/vhosts/(Servername)\;C:\Windows\Temp\) in drupal_pre_render_styles() (line 3437 of C:\Inetpub\vhosts\(Servername)\httpdocs\Drupal-Sitename\includes\common.inc).

I'm not that "special-IT"-Guy, so if you know whats wrong, please be simple.. :(