Hmm, you could try finding your version of the jondome file in your gmod game directory: steamapps\common\GarrysMod\garrysmod\download\maps
If you do find a ttt_mc_jondom .bsp of some sort there, delete it (or just move it somewhere outside of your game hierarchy if you want to keep it), then next time you try to join on jondome it should download it for you via the fastDL.
Usually this bug happens because the client has an older version of the map and it tries to use that one. This bug especially happens if you downloaded the map manually from workshop at some point.