Virtual Center service won't start - 404 Not Found

Hello,

I've not seen this one before so I'm hoping someone else has as I can't find anything on Google.

The VirtualCenter Server service on our vCenter is refusing to start.  I've tried a couple of times and have also rebooted the server without any progress.  The vpxd log shows the following at the end:

2013-05-07T12:44:50.668+01:00 [03352 info 'authvpxdMoSessionManager'] [SSO][SessionManagerMo::Init] Admin URI set to: https://vcenter.intranet.xxx.co.uk:7444/sso-adminserver/sdk

2013-05-07T12:44:50.668+01:00 [03352 info 'authvpxdMoSessionManager'] [SSO][SessionManagerMo::Init] Downloading STS Root certificates ...

2013-05-07T12:44:50.684+01:00 [00308 info 'Default'] Thread attached

2013-05-07T12:44:50.699+01:00 [03352 warning 'Default'] Closing Response processing in unexpected state: 3

2013-05-07T12:44:50.699+01:00 [03352 warning 'VpxProfiler'] ServerApp::Init [TotalTime] took 4890 ms

2013-05-07T12:44:50.699+01:00 [03352 error 'vpxdvpxdMain'] [VpxdMain] Failed to initialize: Invalid response code: 404 Not Found

2013-05-07T12:44:50.699+01:00 [03352 error 'Default'] Failed to intialize VMware VirtualCenter. Shutting down...

Version is 5.1

I'd be grateful if anyway can point me in the right direction,

Thanks.

Steve