May have found an issue, but it will take time to remediate.

So every now and then outbound federation to any server can fail - plenty of good reasons for that. There is a retry mechanism, but it has (again) failed. Normally it breaks more things, which is what made this so obscure. I have reset the retry mechanism, the "activities behind" counter for slrpnk.net is dropping. ~70k to go. At this rate it should (fingers crossed) catch up in the next 2-3 hours.