During my current playthrough, Weylin is nowhere to be seen when entering Markarth, so the Forsworn Conspiracy quest won't start. The first time it happened I thought it might be glitching so I loaded an earlier save and did some other stuff, but I get the same issue even after several days. I can live without that particular quest but it has never happened before and it would be nice to be able to pinpoint what it is that causes it. Googling didn't help so it seems to be a rare issue. Also, I've checked Weylin i TES5Edit, but everything seems normal (only ASIS touches him and it only adds a couple of perks).

Has anyone an idea what I should check?

Have you tried teleporting to him with the console? Or spawning him next to you?

No, I'll try both. Will be interesting to see where he is. And perhaps the quest will start if spawning him at the marketplace. Still wonder what might cause it.

EDIT: He was at the Warrens. Spawning him at the market place made him attack Margret as normal, but after he was killed Eltrys never turns up - so the quest seems to be messed up somehow.