As Prussia I tried to get some Colonial Penetration. Send a trader to several regions on the African south west coast. One of the regions got 5% CP. (The lower one, Namra?).
After this I could place a Trade Post there. While preparing the turn, I actually could, and having the Trade Post icon active, the mouse pointer advised me that "this is a valid structure". After running the turn though, I got the message in the log: "this is not a valid structure in this region", and the Trade Post was not placed.
This stayed this way (got to admit that the trader was still active - is this a problem? It does not state so anywere).