Hi Andre
Out of curiosity, what OS is your server, and how much memory is installed?
In diagnosing this problem (you're not the first to see it), we've been able to determine that the OS X memory allocator behaves differently from Linux's, and the difference is amplified by our specific pattern of memory usage. It's quite an interesting state of affairs, probably worthy of a nice blog post some day...
-dre
Thanks for some more comments. We are using a Mac OS X Server Lion 10.7.4 (11E53) running on a Mac Pro (2.8 GHz Quad-Core Intel Xeon with 8GB RAM). we are using the osx server incl caldav in a commercial environment and this issue is very very critical for us. Would be nice if we get any information/hint regarding this problem. So looks like, we have this problem since version 10.7.4. but we are not 100% sure. Let me know if you need more information, debug loggings or whatever you need to find the reason why we are running out of memory. Many Thanks Marko