Warning from realpath() when open_basedir restriction is in effect

Hi,

when viewing my webform, I get 6 identical errors of the form

 warning: realpath() [function.realpath]: open_basedir restriction in effect. File(/) is not within the allowed path(s):
 (/is/htdocs/user_tmp/blabla:/tmp:/dev/null:/dev/urandom:/bin:/usr:/is/default.errors:/is/htdocs/blabla) in
 /is/htdocs/blabla/www/includes/file.inc on line 188.

where blabla is some lengthy generated path to the webspace.

Except for the occasional update, I have not changed the site recently, so I can only assume the hoster changed something.
Funny fact: It seems this error only emerges when looking at the form as admin - as less priviliged user, it looks just fine. The site is using Admin Role and Administration menu besides ~15 other modules, all up-to-date.

Similar issues have been mentioned before, but it seems they boiled down to wrong file system settings. Mine seem to be valid, though, the status page is happy. The issue seems to be that someone wants to access "/".

I like asking questions the smart way, but with this being my first Drupal issue, many things might not be obvious to me - please tell me what you need!

Cheers,
Dennis