I have seen this problem with inability to ticket an otherwise viable itinerary with LH or LX segment.
In my instance I was trying for YYC-DEN-EWR-MUC on UA connecting to MUC-ARN on LH. The LH segment would not ticket on either UA.com or via the phone.
I have learned from another FTer that LH and LX revenue management systems are configured with an over riding rule that reject low segment revenue. In my case I was trying for a UA issued ticket that was $1100 and LH rejected their portion of ticket. The fix was to book in a higher fare class for $1500 with the difference attributable to the LH segment.
My overall concern is that LH and LX revenue management system rules are creating a bait and switch scheme where the lower fare is offered, but can never be ticketted.