Weird error messages

Hello there!

I have a problem regarding my Drupal installation. It was running perfectly on my old webhosting account from Hosteurope. Now I got my own dedicated server based on Windows Server 2008 R2 and copied the installation directly on the new server which is running on IIS 7.5. Now the existing installation is causing problems: as a guest, the site works just fine and without a problem. But while logged in, many error messages pop up and the backend does not really work 100% fine.

Short info: my web server is already configured correct and runs a fresh Drupal installation without any problem. Also phpBB works perfectly. Only this existing installation is really annoying me. =/

I just upgraded to the latest version 7.18.

This pops up when logging in:

Warning: tempnam() [function.tempnam]: Unable to access /is/htdocs/user_tmp/wp1123684_2ZA9AM1FIE in drupal_tempnam() (Zeile 2406 von C:\inetpub\xxx\includes\file.inc).
Warning: file_put_contents() [function.file-put-contents]: Filename cannot be empty in file_unmanaged_save_data() (Zeile 1903 von C:\inetpub\xxx\includes\file.inc).
Die Datei konnte nicht erstellt werden.
Warning: tempnam() [function.tempnam]: Unable to access /is/htdocs/user_tmp/wp1123684_2ZA9AM1FIE in drupal_tempnam() (Zeile 2406 von C:\inetpub\xxx\includes\file.inc).
Warning: file_put_contents() [function.file-put-contents]: Filename cannot be empty in file_unmanaged_save_data() (Zeile 1903 von C:\inetpub\xxx\includes\file.inc).
Die Datei konnte nicht erstellt werden.

Anyone knows how to fix that? Many thanks in advance!

~Vash