As a sanity check....
It's not enough to be "trading for ages" -- the route has to actually run without interruption until it 'finishes' and you reassign the trader. Unfortunately Nidaros is not in the display, but you can see that Kandy, Amsterdam, Zanzibar, and Buenos Aries have the trading post icon next to their name. Does Nidaros?
If there's some strange reason you can't route through Nidaros, then Xi'an ought to work.
But if you really do have a trading post in Nidaros, I think this is a bug and should be posted to the bug reports forum.