Same thing just hapenned to me, i respawned the darkrai using horon for the second time.
I didn't fight right after i spawned out of the darkrealm. I went to get my catching team with a sync, (no miracle eye or foresight whatsoever)
I went back to murky town to get back to a major city for buying extra ultra balls.
Came back to murky town, used an escape rope to get back to the closest pokecenter from darkrai.
I went to darkrai and i got that illusion message.
sucks hard especially after i used the respawn quest already...