https://blog.mondediplo.net/spip.php?page=backend not a valid feed ?

Hi there,

the gwene.net.mondediplo.blog group is linked to the
https//blog.mondediplo.net/spip.php?page=backend feed, and has not been
updated since end of 2018.

I thought that it was because the website now redirects to https, and
tried to resubscribe the https feed under the
gwene.net.mondediplo.blog.https group, but gwene then says that the feed
is not valid.

So I guess that the group not updating is not due to the https
redirection, but to something that changed in the feed itself.

How can I diagnose what in the feed is annoying gwene ?

Thanks,
-- 
Rémi