I have run across this a couple times now were a habitable planet cannot be colonized by both the player and AI. I have done some searches and found a couple posts back in march about this with little feedback.
This is causing a couple problems. The first is the obvious that we cannot colonize them and the second is the AI seems to keep making colony ships non stop to colonize them instead of other ships as a priority.
So I make a colony ship , send it to the planet, it goes on its way to the planet I clicked on, many turns later I'm thinking, well my ship should be there by now, I go to look, and there the colony ship just sitting around the planet. No idle ship warning was triggered. I click on the planet to colonize and the auto move acts like nothing is there to way point to.
I have had this happen once on a small map, and two more times on the largest maps.
Below you can see the AI has got colony ships just hanging around the two planets. The AI will sit around for about 15 turns then leave and come back for another try. This could just be a coincidence but from my observation these planets always seem to be on the right side of the map by the boarder.