Internal Server Error when clicking on iCal links

[gsauthof]

For example, when clicking the iCal links on https://www.mehr-schulferien.de/ferien/d/bundesland/hessen

• https://www.mehr-schulferien.de/api/v2.0/icalendars/location/hessen?vacation_types=school&year=2021
• https://www.mehr-schulferien.de/api/v2.0/icalendars/location/hessen?vacation_types=school&year=2022

I currently get:

"Internal Server Error"

Tested with Firefox and Chromium.

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Btw, I couldn't find any information on what the supported vacation_types are - e.g. is there support for non-school-holidays types, such as special days?