@jabber Weird, I (not on jabber.de) can do omemo with all of my xmpp contacts but not with the one of my wife who is on jabber.de. That is using most clients, libpurple-based ones, miranda-ng. It works fine using conversations or gajim. Encryption is shown as not available. It works fine with other contacts on a self-hosted server using the same clients though.
@spaetz Could you check your SSL Certificate? - https://xmpp.net/result.php?domain=sspaeth.de&type=client (DST Root CA X3 expired!).
@jabber uhh, thanks for the hint, it is a letsencypt cert which is automatically renewed. Will check.