I am encountering the same problem. Here is some data from the debug logs on the server, if it is helpful:
Jan 19 02:03:59 c2s80bbd3ac0 info Client connected
Jan 19 02:03:59 c2s80bbd3ac0 debug Client sent opening <stream:stream> to mydomain.tld
Jan 19 02:03:59 c2s80bbd3ac0 debug Sending[c2s_unauthed]: <?xml version='1.0'?>
Jan 19 02:03:59 c2s80bbd3ac0 debug Sent reply <stream:stream> to client
Jan 19 02:03:59 c2s80bbd3ac0 debug Not offering authentication on insecure connection
Jan 19 02:03:59 c2s80bbd3ac0 debug Sending[c2s_unauthed]: <stream:features>
Jan 19 02:03:59 c2s80bbd3ac0 debug Received[c2s_unauthed]: <starttls xml:lang='en' xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
Jan 19 02:03:59 c2s80bbd3ac0 debug Sending[c2s_unauthed]: <proceed xmlns='urn:ietf:params:xml:ns:xmpp-tls'>
Jan 19 02:03:59 c2s80bbd3ac0 debug TLS negotiation started for c2s_unauthed...
Jan 19 02:03:59 c2s80bbd3ac0 debug Invalid opening stream header ({jabber:client}iq)
Jan 19 02:03:59 c2s80bbd3ac0 debug Sending[c2s_unauthed]: <?xml version='1.0'?>
Jan 19 02:03:59 c2s80bbd3ac0 debug Disconnecting client, <stream:error> is: <stream:error><invalid-namespace xmlns='urn:ietf:params:xml:ns:xmpp-streams'/></stream:error>
Jan 19 02:03:59 c2s80bbd3ac0 debug Sending[c2s_unauthed]: <stream:error>
Jan 19 02:03:59 c2s80bbd3ac0 debug Sending[c2s_unauthed]: </stream:stream>
Jan 19 02:03:59 c2s80bbd3ac0 debug c2s stream for 73.X.X.X closed: invalid-namespace
Jan 19 02:03:59 c2s80bbd3ac0 debug Destroying session for (unknown) ((unknown)@mydomain.tld): invalid-namespace
Jan 19 02:03:59 c2s80bbd3ac0 debug Discarding data received from resting session: <query xml:lang='en' xmlns='jabber:iq:register'/>
Jan 19 02:03:59 c2s80bbd3ac0 debug Discarding data received from resting session: table: 0x80bc1a080
Jan 19 02:03:59 c2s80bbd3ac0 info Client disconnected: connection closed
Jan 19 02:03:59 c2s80bbd3ac0 debug Destroying session for (unknown) ((unknown)@(unknown))
my server is prosody. registration is enabled
allow_registration = true
user can login only after prosodyctl adduser