device enrollment fails bad certificate

I am unable enrolling a Devie in iOS 5.0.2 with my Lion Server 10.7.2.

On the iPhone 4S I can go to the /mydevices/ URL to register the device. In the Devices tab I can see a big blue 'Enroll' button underneath 'This 'iPhone'. It takes me to the correct pane in the Settings app, but when I click 'Install' it shows this error:

The server certificate for "https://[myserver]/devicemanagement/api/device/ota_service"; is invalid.

Also trusting the certificate through the profiles menu in advance doesn't solve the issue. Even when the Trust profile is installed and verified.

I can't see any way to tell the profile manager which cert to use. I have found a log file on the server called profilemanager.log which is obviously reporting things relating to the error, but I can't find any command-line tool that's related to it.

I have no problem enrolling an iMac Lion 10.7.2!

PS I use a signed certificate