I had a problem with buying a Lufthansa connection as well. Web support could not help me. It was very annoying. One comment was - Lufthansa may have not released seats yet - it was a couple months out - really, it shows availability and its not 6+ months out? We ended up taking an alternate all UA itinerary that worked but very frustrating.
Can regular telephone agent purchase the ticket? You can ask them to waive agent fee since it can't be bought via .com or web support?