If you are getting the following error:
"The following error occurred while trying to join the session:
Failed to verify signature on program executable. "
What this means is that for some reason your executables do not have the proper digital signatures on them.
What might help is to delete the version_system.txt from the iRacing installation then the web-site should ask you do redownload and install the system package again.
If you have any problems updating through the web interface, try navigating to your C:\Program Files (x86)\iRacingUpdate and double click the iRacingUpdater.exe
Once that runs and you click Done, go back and refresh your browser.
It should now say you are Up To Date.