If you receive this error the best solution is:
Be patient and try again later. If the issue persists after a few days, then please post about it.
TheTVDB, Trakt and TMDB as well as SeriesGuide are services operated by a small or even one-person staff. If the service experiences issues, the person which can fix it can not always be available (it’s night time, they are working another job or are on holiday). So it can take time until issues are resolved. Please do not expect 24/7 availability like from services of big corporations that can afford a large, around the world support staff.
Thank you for understanding!
Links to learn about ongoing issues and their status:
Note: some countries or internet service providers (ISP) are known to block e.g. themoviedb.org or trakt.tv. In this case it may be possible to switch to another connection (different ISP) or use VPN software (at your own risk).