This make sence.... Sounds like a good idea to deactivate all mods ecept my desired one... Maybe this will fix the Problem.
It might make the problem go away for now, but it won't fix it. The problem is that one or more of your mods are conflicting with each other, OR with the vanilla game, OR with some DLCs you've purchased.
Here's the thing. Before the March patch, you could have two mods adding or editing the same text key, and it wouldn't break like this. Instead, it'd just overwrite the first one loaded (which'd be the one in the vanilla game, if any) with the second. What happened, fairly often, was that mods would accidentally re-define certain existing keys in relatively harmless ways. The most common, by far, was when a custom civilization would use a city name that the game was already declaring. Someone would decide to add a Korea civilization (pre-DLC, I mean), would define TXT_KEY_CITY_SEOUL as "Seoul", and the fact that the game had already defined that same key for the city-state wouldn't cause a problem because both were setting it to the same text string. The mods in question were not technically breaking anything, but they weren't actually working together correctly.
In the March patch, the devs changed it to do what you saw, where the entire XML file containing the second instance would fail to load in case of a conflict. Again, this was most commonly encountered in mods or scenarios that added a custom civilization of some kind. Since many of these have long since been abandoned by their creators and/or superceded by DLC civilizations, they were never repaired to work with the updated game. You can still find the broken mods fairly often, especially if you're the kind of person who uses the in-game Mod Browser to find content instead of scanning these boards to see what's still being kept current.
Now, you CAN fix it fairly easily if you want to use these mods together. If you go into your user directory (that's My Documents/My Games/Sid Meier's Civilization V/ on a Windows 7 machine), there's a "Logs" directory. Inside it are a bunch of log files the game generates; the two you'd care about for this are Database.log and xml.log. The Database.log is the main one you want to use here; it'll give messages like "TXT_KEY_CITY_SEOUL is multiply defined" to let you know exactly which keys are causing the conflict. Go into whatever mod is using that key (Windows has a Search function, after all) and just delete that entry from the offending XML file. Repeat as necessary, until the logfile reports no more errors like that.
NOTE: If Database.log isn't being created, then you don't have logging turned on at all. In your user directory is a file "config.ini". Open that in a text editor, and search for LoggingEnabled = 0. Change it to a 1. While you're at it, the top of the file has EnableTuner = 0; change that one to a 1 too. There will be a few other logging options within the file; change whatever sounds interesting to a 1.