Uh oh... did I finally get flagged as the dreaded quadruple ssss?

DH just checked in online for our return flight from EDI tomorrow morning. No problems.
When I went to do online check in moments later, I got a message similar to 'We are unable to check you in. Please see a Delta agent at the airport" (not exact wording, but very close).

Is there anything else that might cause this?
I had no problem checking in on the outbound flight (and in case it matters, I have "behaved myself" while here ).

The boarding time is also set for 55 minutes prior to departure time. We don't remember seeing this before, but given our penchant for arriving extra early (lest there be delays en route to the airport or airport delays - er, like I might be facing?), this shouldn't be a problem.

If it IS the dreaded "quadruple letter", are they likely to need any special docs that we might not have with us? Or mostly answer a bunch of questions? Or...?

Many thanks!

GC