PHP Warning: Operation not permitted

[Sönke Ebert]

Hi,
I discovered an error in my apache-error.log:
[error] PHP Warning:  rename(/var/tmp/imapcache6pXRbf,/kolab/var/ 
kolab/www/freebusy/cache/architekt-miro^de/mirco^meier/ 
Kalender.imapcache): Operation not permitted in /usr/local/kolab/var/ 
kolab/php/freebusy/freebusyimapcache.class.php on line 191

This message appears periodically when a outlook client is logged in.  
It probably tries to read the freebusy information.
My system is FreeBSD 6.2 running Kolab 2.1 final

Any ideas what could be wrong?
Thanks