Aenlic, I know what would have caused that error. It's the fact that I didn't change the name of the units and buildlings texts from v3, so they are incompatible with the old v3 campaigns. You did the right thing by making a clean install, though for anyone else reading this who has the same problem, you can correct it by simply deleting the old medmod campaign files.
They are located in the Medieval - Total War\campmap\startpos folder, and have the MM prefix in their names. Sorry about that.

Alexius, I would recommend reviewing the installation instructions, especially if you are using a non-English verison of the game. I think you just installed the Texts first instead of the Graphics, however.
Just re-install the Texts and that should fix the problem.

As for the strategic map, I have just not gotten around yet to altering the provincial names. When I reverted back to the old names, that was just in the core text files, which should not affect what you see displayed in the game.