This error typically means there is something wrong with a track file.
If it's only happening with one particular track, you can navigate to C:\Program Files\iRacing\Tracks and delete that one problem track.
Then go back to the members site and redownload and install.
If that doesn't work, go back and delete that one problem track and also the tracks.dat and version.txt file in the Tracks directory.
If it's happening with all tracks, delete the entire tracks folder and reinstall all of the tracks.