There will be multiple reasons. I suspect this mostly co.es down to lemmy.world being victim of a DDoS recently.

Additionally the Lemmy code itself now has mechanisms to stop instances forever trying to send content to offline instances. The code isn’t perfect and has been known to have false positives. I’ll check tonight if this may be the case here, though this was supposed to be addressed in Lemmy 0.18.4.

There are other possibilities, but these are most likely in my mind.