The problem is most likely caused by the history of their local copy of the project getting corrupted - system crashes can cause this.
We normally send steps like this to users:
- Backup your project
- Restore the backup to a new project name
- Delete your local copy of the project
- Retrieve the project from the server using send/receive
- Use Tools/Compare Texts to find changes between the restored project and the project created in step 2.
We did try to make Paratext 9.2 able to recover the project from a backup that is created at the end of each successful send/receive of the project.
They can try the “Paratext > Advanced > Diagnostics > Verify project repositories…” command on the main menu
John+Wickberg
Paratext Support