Hi,
There are many reasons for "Failed to Save document" error.
It might be due to (but not limited to these):
1. Network failure while we are working on a report.
2. Any database field used in a formula is not available (including those from stored proc).
3. If working on a report on a machine a remote connection and your PC goes into hibernation or temporarily lose connection, then also report fails to save.
Few Suggestions:
1. Do a verify database to make sure you have updated database info in the report.
2. If you are using Oracle database as backend and changing the database schema using set location option, then update each table from both schemas using set location option.
3. If your report is having sub reports and you are in preview mode, and trying to save report, try closing the previews for sub reports first and then try to save it again.
4. If you are on remote connection, try saving the report frequently, so that minimal changes will be lost if you lose a connection.
Hope one of these helps you to resolve or get a hint on your issue.
Thanks,
Raghavendra