Summary
The most likely cause of this problem is insufficient file permissions. More specifically, the user that's running the CompleteFTP Service must have Full Control of the directory, C:\ProgramData\Enterprise Distributed Technologies\Complete FTP\, and the file, config.db, which is in this directory.
Details
Service Account
By default, the CompleteFTP Service runs under the local SYSTEM account, but unless you're 100% certain that this hasn't been changed, it's best to start by checking this. To do this, open the Windows Services console; double-click on CompleteFTP Server; click on the Log On tab. If Local System account is selected then the username is SYSTEM, otherwise it will be shown next to This account:
Program Data Directory Permissions
To check the permissions of the directory, open File Explorer; navigate to C:\ProgramData\Enterprise Distributed Technologies\; right click on Complete FTP; select Properties from the list; select the Security tab; click Advanced; select the Effective Access tab; click Select a user; enter the name of the account found above; click OK; click View effective access; verify that Full control is checked.
Configuration File Permissions
To check the permissions of the directory, open File Explorer; navigate to C:\ProgramData\Enterprise Distributed Technologies\Complete FTP\; right click on config.db; select the Security tab; click Advanced; select the Effective Access tab; click Select a user; enter the name of the account found above; click OK; click View effective access; verify that Full control is checked.
