Hi Stephen,
Welcome.
Is this for Windows or macOS ? Which version.
Assuming Windows,
Lets start afresh.
Please disable any Antivirus and firewall.
Please use Windows to uninstall FMS Data Manager. Specifically use FAQ - Charts desktop installer fails immediately
If they exist, please delete folders:
c:\ProgramData\Navigraph\FMSClient and c:\Program Files (x86)\Navigraph\FMS Data Manager.
Please download and install latest FMS Data Manager .
Please check X-Plane 11 path in Settings page is correct.
In Addon List select and Remove entry X-Plane 11 (11.50+). You should have entry X-Plane 11 (11.50+) again. Press Scan and Save.
In Addon List what happens when you select and Update entry X-Plane 11 (11.50+)
Please let us know how you get on.
Cheers
Ian