I know, I tried it several times. Still same outcome. Thats the reason why I would try it via the airport to avoid stress.
I also ask them to send the routing to the "backoffice" to "manual" book the flights. Still same answer.
What is the most bizarre part, the agent seems to prefer UA flights via the LH flights (only 1,5h later/ same routing/ and still connection under 24h).
I also checked availability in Expert flyer +9 Seats in every booking class.