There could be more reasons and you can't typically spot this rare error. In general it means the service is down for some reason, what mostly happens for seconds or few minutes in the worst case.
1️⃣Please just be patient and if necessary, reload the page
2️⃣If the problem persists, please check our status page to find out whether there is any outage currently. You can even subscribe there to get an e-mail when the problem will be solved.
3️⃣If the problem persists and there is no outage currently announced on the status page, please reach our support and don't forget to include as much details as possible.