Same thing as I reported last week:
https://www.flyertalk.com/forum/luft...ites-apps.html

The multi-city tool doesn’t work very often. I called the web support number and they told me to use Chrome instead of Safari or failing that to call reservations. Telephone reservations said the flights I wanted were not available, despite the multi-city tool showing me plenty of availability. I gave up and booked 2 return flights nested to cover 2 future trips. I am BA refugee who naiively thought LH IT would be better. Well, it is generally, but not in this case.

I still suspect its a married-sector or point of sale/commencement issue within Lufthansa’s systems.