Several people have posted about these errors when attempting to book multi-stops. I see three workarounds: (i) call [demand waiver of phone booking fee due to buggy web page); (ii) book through a reputable OTA such as Expedia; or (iii) try to go to the old web interface (to the extent still accessible):


https://book.lufthansa.com/lh/dyn/ai..._OVERRIDE=TRUE