@ldwvii https uses encryption to protect your data while traveling across the internet.
to do this, the receiving host must have registered to support that encryption.
it does this by recording it’s url and encryption type with a certificate authority ( not just anybody can be an authority), they give back a file with a key, with a specific lifetime. ( so u can’t have old sites with bad security hanging around forever)
well the authorities also have expiring rights.
in this case the authority’s certificate expired. (it’s a chain ) … and there was a bug in electron that didn’t handle it correctly
now, in your case u use an unprotected url ( no longer allowed as a standard)http://kalender.link/ical/best
no ‘s’ on http and it was redirected to secure (https)… maybe the redirect sent to the wrong place…