Been there, exact same situation, swore like hell and hit the PC a few times, then restarted from a VERY outdated old save game because no-one on the forum or anywhere accessible via Google had a solution. Didn't work, eventually had to load someone else's savegame of a gamestate AFTER the FUBAR'd mission. There is no known solution apparently, went through every suggestion/trick/cheat/console/tweak etc even remotely applicable and only ended up screwing up the rest of the game further. If you're gotten to the error it's too late to fix as the cause seems to be well-hidden/embedded in the game itself, or possibly caused by a mod. I have a crapload of mods installed and it's impossible to determine which one or if it's the game engine itself without replaying the game from start as vanilla, then with multiple combo's of mods.