It’s likely a SSSS (no special documentation or action needed on your part, just a little extra patience for extra security screening).

Could also be a global upgrade that’s cleared but not properly reissued; if you were waitlisted for an upgrade of some kind, it’s worth calling to see if the ticket needs to be reissued.

But those two are pretty much the extent of the reasons for this error, and SSSS is the most likely. Don’t panic - it’s annoying, but routine.