An educated guess is your savegame has been corrupted. You should be able to fix it by going into your most recent save and removing 20_GoatCity.dxs from the folder. Don't delete it though, just move it somewhere else, just in case. If it works, then you can delete it after checking. This has the side effect of resetting any changes you've made to the Goat City level, reviving any non-critical character you've killed and respawning any item you've picked up, relocking doors you've picked etc.
My concern is that judging from your error message, it may be the Goat Templae level that's fucked up, which is where you made the save, right? In that case you could be out of luck, and you may have to go back to an earlier save. In that case, I'm sorry
If you'd like to try to teleport yourself to Goat City before doing anything else, the command is "travel 20_goatcity" without the quotes.