Well it hapend again.
The same user is having the same issue.
I tried step 2.
I'm on lion 10.7.5 now so I edited /etc/caldavd/caldavd.plist.
Added this key to the caldavd.plist and restart calendar server:
did not do this.
From then on evey request/response is logged as a separate file in /var/log/caldavd/accounting/HTTP
I tried it without making the accounting directory and with manualy making the accounting directory. I then made HTTP directory for good measure.
Thanks for your help,