I have a problem with the XMPP-connection to mailbox.org

I have a problem with the XMPP-connection to mailbox.org (tested on different PC’s, different networks, no AV, no firewall …) I always get this message/report:

09:16:29 …@mailbox.org [XMPP]  MailClient.Storage.Application.OperationException: Unexpected exception —> System.Exception: Unexpected exception —> System.NullReferenceException: Der Objektverweis wurde nicht auf eine Objektinstanz festgelegt.
09:16:29    bei MailClient.Protocols.Jabber.JabberManager.vcard_Retrieved(Object sender, IQ iq, Object data)
09:16:29    bei jabber.connection.IQTracker.TrackerData.Call(Object sender, IQ iq)
09:16:29    bei jabber.connection.IQTracker.OnIQ(Object sender, XmlElement elem)
09:16:29    bei jabber.connection.XmppStream.OnElement(Object sender, XmlElement tag)
09:16:29    bei jabber.client.JabberClient.OnElement(Object sender, XmlElement tag)
09:16:29    bei jabber.connection.XmppStream.jabber.connection.IStanzaEventListener.StanzaReceived(XmlElement elem)
09:16:29    bei jabber.connection.SocketStanzaStream.m_elements_OnElement(Object sender, XmlElement rp)
09:16:29    bei jabber.protocol.ElementStream.FireOnElement(XmlElement elem)
09:16:29    bei jabber.protocol.AsynchElementStream.EndTag(Byte[] buf, Int32 offset, ContentToken ct, TOK tok)
09:16:29    bei jabber.protocol.AsynchElementStream.Push(Byte[] buf, Int32 offset, Int32 length)
09:16:29    — Ende der internen Ausnahmestapelüberwachung —
09:16:29    bei jabber.protocol.AsynchElementStream.Push(Byte[] buf, Int32 offset, Int32 length)
09:16:29    bei jabber.connection.SocketStanzaStream.bedrock.net.ISocketEventListener.OnRead(BaseSocket sock, Byte[] buf, Int32 offset, Int32 length)
09:16:29    — Ende der internen Ausnahmestapelüberwachung —

Of course I’ve installed latest release of eM Client and full updated Windows 10. Any ideas?