Can't access Niko's Mine Encampment after migration

I was playing in a Private League, but migrated a character to Trade (a single one; didn't use the "Migrate All" option).

However, there seems to be a bug with Niko. I can't visit the "Mine Encampment"; it's giving me the error "You do not have the required quest state", no matter if I try to teleport through a waypoint or through Niko himself.

I went to Act 4's Highgate, but he's not giving me a quest. I fully roamed "The Mines Level 1" zone, but there wasn't a single Sulphite node to trigger him.

I have both "Niko's Fuel" and "Niko's Mine" quests as incomplete.