Their API (and the site) don’t have English information for this series. Sonarr requires English information for the series to exist on TheTVDB. If this series was previously working its likely that they removed non-English information from the English information and its caused this issue.
You may be able to add the English information for the series.
If there is English information available for this series you could add it, but re-adding the Dutch information is likely to get it removed again.
Likely because someone added the dutch information to the English page, which is actually against TheTVDB’s rules, I’d expect it to be removed when they notice it.