Have you tried calling JL to book?
No, haven't tried yet. I am actually more interested in understanding the math behind it (that's why I bought the KVS tool diamond level

).
I could understand if the LX 645 flight would not be bookable with this JL tarif on any day because of some fare rules restrictions or minimum connection times. I could also understand if this flight would not be bookable because the booking class is not available on that day for that flight. But I absolutely do not understand why I cannot book that flight on that particular day whereas it works perfect on the next day with exactly the same connections!