Interpret error-log line

[DSF]

Very often I find this line in my server error-log.
What it this?

[Tue Nov 23 18:12:27 2010] [error] [client 91.210.104.45] File does not exist: /var/www/qwertz/my-domain.com/httpdocs/register, referer: http://www.my-domain.com/css.php?css=login_bar,node_category,node_forum,node_list&style=2&d=1290187527

 

[Mike]

Perhaps look for an image that's referenced in that css.php URL that makes reference to "register" or is possibly an empty image URL. Though with friendly URLs enabled, that should be a valid URL.

I'd have to see the actual site to say for sure.