the problem seems to be that (a) Delta Chat does not support password encrypted keys and (b) it is hard for enigmail to generate such keys with gpg
but iirc, if your key does not have a password, it works if you answer gpg’s question " Passphrase: This key (or subkey) is not protected with a passphrase. Please enter a new passphrase to export it." with “OK”. Otherwise garbage is exported …
this is the state from about half a year ago, written down from my memory, so things may be quite different today.