So what is this about? The sync being slow or the sync getting stuck?
I am sorry you are frustrated but it's unlikely that it's because of the app.
When there is the "Are you trying to sign in to Diarium?" prompt, this is likely in a browser window, right? if nothing happens when clicking on it, then the browser window is not correctly redirecting back to the Diarium app. Nothing I can do against that. I would recommend setting Google Chrome as your default browser.