Do you see any requests from the Playbook hitting your server when you try to configure the account? Look in /var/log/caldavd/access.log.
Are there separate fields for entering user name and server address? Is your CalDAV server on hosted on a machine with a DNS name that is the same as your email domain (e.g. example.com vs calendar.example.com)?
What I can see from the error logs is:
09:10:29+0000 [Uninitialized] [twistedcaldav.mail.IMAP4DownloadFactory#error] IMAP factory connection failed
2012-12-11 09:10:59+0000 [-] [mailgateway] 2012-12-11 09:10:59+0000 [Uninitialized] [twistedcaldav.mail.IMAP4DownloadFactory#error] IMAP factory connection failed
2012-12-11 09:11:29+0000 [-] [mailgateway] 2012-12-11 09:11:29+0000 [Uninitialized] [twistedcaldav.mail.IMAP4DownloadFactory#error] IMAP factory connection failed
The only things in the access logs are succesful iOS actions.
My server is a Mac Mini running OSX Server. It uses a no-ip redirect, which doesn't prove a problem for the iOS / OSX devices. I have tried accessing via IP address whilst on the LAN as well.
The setup is such that I am only running Wiki, file sharing and Time Machine. I added calendar so that we could share events and see what we are all doing without the need to permanently share our iOS / OSX calendars.
Thanks - it would appear that the PlayBook is using a non-standard configuration!
Every other device just seems to link. So I try all the defaults - but nothing seems to get it to work. iPad and iPhone all work first time. Windows Phones will only link to email account calendars (Google / Live), looks like Android will work through various apps.
Thankfully most users use OS X or iOS devices!
Will look if I can sync with Google Calendar from the server...