Another user is currently backing up this company file. Unable to sign on at this time; please try again later.
Troubleshooting: alert and error messages > Alert messages for company file and networking >>
Mac OS X Users - IMPORTANT!
Situation
You attempted to open a company file that is currently being backed up. Only one person can be accessing a company file during the backup process.
Solution
Wait until the backup process is done, then open the company file.