NAPTR lookup through proxy - "No address found"


I’m trying to get Jitsi SIP working with the Deutsche Telekom Cloud PBX solution.

It says to configure:
SIP-Domain: tel.t-online. de
Proxy-Server: hpbxsec.deutschland-lan. de:5061

If I enter that configuration data, I get the following error:

impl.protocol.sip.ProtocolProviderServiceSipImpl.register().443 No address found for ProtocolProviderServiceSipImpl( (SIP))

hpbxsec.deutschland-lan. de has only a NAPTR entry pointing to _sips._tcp.hpbxsec.deutschland-lan. de

host -t SRV _sips._tcp.hpbxsec.deutschland-lan. de delivers

_sips._tcp.hpbxsec.deutschland-lan. de has SRV record 10 0 5061 f-epp-652.edns.t-ipnet. de.
_sips._tcp.hpbxsec.deutschland-lan. de has SRV record 30 0 5061 h2-epp-652.edns.t-ipnet. de.
_sips._tcp.hpbxsec.deutschland-lan. de has SRV record 20 0 5061 d-epp-652.edns.t-ipnet. de.

Any idea whats wrong here and how to fix it?

(note: space before .de due to link posting limits)