I have accidentally added a second colony to mercury, so I deleted it again. While in the main Window (with the summary view) the correct colony got deleted, in the ships orders something went astray: There the colony with the 0 Pop stayed, while the other (real) colony went missing. So I could only give my ships the orders to fly to the empty colony, making me unable to load or unload anything at the actual colony, plus produced an error when they wanted to load or unload something, because then they obviously noted that the colony they were flying to wasn't there anymore. I solved the problem by deleting the old colony too and recreating it in SM mode, however, both freighters of a civilian shiping line are waiting in Mercury orbit to trade their goods ever since (more than a year ago), and don't move anywhere anymore (although by now there's no colony at all anymore, because I abandoned the project as too costly).