The first thing to try is the automatic repair:
Click Tools
Select Check Job
Preview the report
It should pop up a message saying "Errors have been fixed" before giving the print preview of the Check Summary.
Close the preview and repeat the process, this time it should not say that errors have been fixed, it should go straight to the print preview.
You should no longer be getting the error.
If Check Job crashes, does not report that it has fixed anything, says that it has fixed errors more than once or does not correct the error please continue with a support member for further assistance.
Project corruption is usually caused by either poor network connectivity (especially VPN) or having two people in the estimate at one time.