Lemmy.ml has been overloaded today due to the influx of reddit users angry about API changes impacting 3rd party apps. Some context for the overload is available in https://lemmy.ml/post/1147770.

It seems reasonably likely that random things were failing during the peak of the overload. I believe they added capacity not long after you posted this and that things are at least temporarily in better shape. You might want to retry (or wait to create new communities until things settle a bit).