VPN on OS X Server working, but no visible changes

Hello,

I have just installed a Mac Mini with OS X Server 5 running and have successfully enabled the VPN server. I have verified that I can access the VPN through an iOS device from outside my network. The OS X Server console responds as expected ("Call incoming ... Connection established ...").

However, when I actually make any changes to, say, calendar entries, they do not seem to arrive on the server. I simply never see any changes whatsoever.

What I am seeing in the log is this:

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Jan 19 12:05:30 praxisserver AddressBookSourceSync[42366]: [CardDAVPlugin-ERROR] -50 <Unknown error -50> from KRBAcquireTicket <HTTP/praxis-server.local@LOCAL Bijan@LOCAL>

Jan 19 12:05:30 praxisserver AddressBookSourceSync[42366]: [CardDAVPlugin-ERROR] -[CDXAccount getKerberosTicketWithConnection:]:<NSURLConnection: 0x7fa0b05c7bf0> { request: <NSMutableURLRequest: 0x7fa0b05c4880> { URL: https://Bijan@praxis-server.local/principals/__uids__/E6AEE748-FD09-4445-A359-98 628C5644B2/ } } returning NO because can't get Kerberos ticket acquired for host: praxis-server.local user: Bijan

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Am I right in thinking that there might be something wrong with Kerberos authentication? Do I need to turn that off somewhere?

Thanks for any help!