These are triggered if some of your content (specifically a track) are not up to date, did not update properly, or have become corrupt. You will need to remove and reinstall the pieces that are in error.
Steps to Fix:
1. Go to your installation folder (generally C:\Program Files (x86)\iRacing).
2. Go to the "tracks" directory.
3. Delete the "tracks.dat" and "version.txt" files.
4. Delete the directory for the track you were trying to use.
5. Refresh the web site and it will tell you that you have updates available.
6. Install the (now) missing track and the shared track data.