Mark,
Sorry to hear of the issue.
I'm thinking you are talking about the GPS Track Upload function in the MapRun Console (
https://console.maprun.net/#/) or maybe the preview version of the console (
https://consolepreview.web.app/#/).
The "choose track file" button does not reach out to the server, it only tries to open a file selection dialog within your browser. Maybe there is a permissions issue? Restart your PC?
You could open Developer Options in Chrome to see if you can see an error message. That is: Chrome > View > Developer > Javascript Console.
Maybe you could send some screen shots to support@maprun.net.
Peter