Content server will only start in debug mode

Something has recently happened and the content server will now not start, There is simply the message:

calibre, version 3.35.0
ERROR: Failed to start Content server: Could not start the Content server. Error:

Client socket not connected in 1 second(s)

I'm using Windows 10, latest updateI have reinstalled calibre, switched off the firewall and antivirus, deleted the metadata files - but always get the same message. Then, as a last resort I started in debug mode and the server works fine in that mode, but not when started normally. Can anyone help?