When something goes wrong
How the app checks your connection, and what the notices mean.
Connection checks
Every connected account is tested about every 15 minutes, and again whenever an upload fails. A session with a problem says so at the top of the page and on the session itself, in words rather than only in colour.
The notice names what went wrong and what to do about it, whether that is signing in again, ticking a permission you missed, turning on the Fitness API, or waiting out a slow patch at Google.
The usual fixes
Reconnect fixes most of it. It signs in again on the same session and keeps your settings and history.
If Google turned the app away rather than the account, the problem is in the site's own Google setup, which the people running this site need to fix, and the notice says so.
Uploads that fail are retried, and a run that hits a wall stops and records why in the log.