Looks like the underlying problem was caused by Norton Antivirus Data Protector.
I deleted the old 'C:\Program Files\Downlord's FAF Client' folder and down loaded the latest FAF client installer from https://www.faforever.com/
When I tried to run this I got a "Data Protector blocked a suspicious action by faf_windows-x64_2022_4_0(1).exe" message.
I managed to get Norton to exclude this process and then everything seemed to run okay.
2022-04-29 (1).png