I reported this same issue today under ticket MCPE-116314. It now shows resolved as being tracked as a duplicate to this ticket. I only have this issue with one downloaded game: Mars Protocol. All other games are loading without issue. I saw a thread about disabling multiplayer and that did not resolve the issue. I believe it to be an problem within the add ons because when loading resouce packs there is a slight skip in the screens and directly after is the "Unable to connect" error.
Not sure how all this process works but I see above it says bug is resolved. What timeframe should I expect for functionality? I ask because the game still does not load, there is no change in the error. I reinstalled with no success. I turned off multiplayer because of the additional comments in this thread with no success.
It may also be an issue with the log in status to Minecraft. I have been able to load the Mars Lander saved game after exiting Minecraft (check to make sure your NAT is Open), then go back into Minecraft. I have problems with double NAT in the past so I am always checking it with Minecraft.