We are seeing this as well. Reported by Chrome users - we have been unable to reproduce the error ourselves.
In our case, the user is active on our site (with HTTPS active) and when they then make the next request they will get the error. It is occurring on an AJAX request only, so we are not able to see the actual full error reply.
The browser is returning an error 525, which is triggered because the AJAX request is failing.
We are seeing it at a variety of times. Previously we had no way to associate the exact user request in the logs. So we know the error was happening at the client side, but could not associate it to an actual request.
By enhancing some error details we should now be able to associate a user error to an exact request, this only happens if going through Cloudflare. It does not happen if the user goes direct to our servers.