I know i sound like an idiot, but I'm have a real problem with this.
First off, I have 2 gtaIII's on my PC right now one is for singleplayer modding and the other specifically for LU.
It used to run just fine until I put a mod onto the wrong folder. Then this guy pops up:

Error: Your client failed a CRC check on data//default.ide

 I then did what everyone told me, I deleted the gtaIII folder for LU and reinstalled it, but still, it did not work.
I removed the modded gtaIII, then tried again but still, the same error pops up. Even with no mods, I cannot play LU anymore. Somebody please help!
The same happened with LanHikariDS. Some servers work, but others, specifically polaski and GTA-MP.cz don't.
I'm guessing it has to do with certain player's region, as the error is inconsistant.