Another user is currently backing up this company file. Unable to sign on at this time; please try again later.

Troubleshooting Alert Messages Overview > Alert messages (in alphabetical order) > Another user...
If you haven't already done so, we suggest you read the Troubleshooting Alert Messages Overview.

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.