This morning our freshly installed SharePoint Server 2013 server was refusing to let me save a simple form with a Lookup column to another list. Using the standard insert/edit form, the error message read:

Error message from insert/edit form: *The server was unable to save the form at this time. Please try again.*

Error message from insert/edit form: The server was unable to save the form at this time. Please try again.

When using the quick edit mode, the error message was:

Error message in quick edit mode: *This service isn't available right now. Click here to try again.*

Error message in quick edit mode: This service isn’t available right now. Click here to try again.

The fix turned out to be a iisreset on the SharePoint server.

Update: It looks like the culprit is the Security Token Service, more info over at Sharepoint2013.me.

I hope this helps somebody else save a few hours of head scratching.