I don't think the issue is with the website though it's not impeccable.
The issue is with the routing you're trying to get. See I searched on EF for through fares LHR-HVB on EK and could not find any. Searching for DXB-HVB came with plenty but even those have the following routings (haven't checked for exclusions):
1. DXB-EK/QF-BJS/BKK/CAN/HKG/JKT/KUL-SIN-BNE-EK/QF-HVB
2. DXB-EK/QF-DXB-EK/QF-BJS/BKK/CAN/HKG/JKT/KUL-SIN-BNE-EK/QF-
HVB
3. DXB-EK/QF-DXB-EK/QF-HKG/SIN-QF-BNE-EK/QF-HVB
4. DXB-EK/QF-DXB-EK/QF-HKT/MNL/OSA/SEL/SGN/SHA/TYO-SIN-BNE-
EK/QF-HVB
5. DXB-EK/QF-DXB-EK/QF-MLE-CMB-SIN-BNE-EK/QF-HVB
6. DXB-EK/QF-DXB-EK/QF-SIN-BNE-EK/QF-HVB
7. DXB-EK/QF-HKG/SIN-QF-BNE-EK/QF-HVB
8. DXB-EK/QF-HKT/MNL/OSA/SEL/SGN/SHA/TYO-SIN-BNE-EK/QF-HVB
9. DXB-EK/QF-MLE-CMB-SIN-BNE-EK/QF-HVB
10. DXB-EK/QF-SIN-BNE-EK/QF-HVB
I also looked up the routings for EK fares HVB-LHR:
1. HVB-EK/QF-BNE-QF-HKG/SIN-EK/QF-DXB-EK/QF-LON
2. HVB-EK/QF-BNE-QF-HKG/SIN-EK/QF-DXB-LON
3. HVB-EK/QF-BNE-SIN-BJS/BKK/CAN/HKG/JKT/KUL-EK/QF-DXB-EK/QF-
LON
4. HVB-EK/QF-BNE-SIN-BJS/BKK/CAN/HKG/JKT/KUL-EK/QF-DXB-LON
5. HVB-EK/QF-BNE-SIN-CMB-MLE-EK/QF-DXB-EK/QF-LON
6. HVB-EK/QF-BNE-SIN-CMB-MLE-EK/QF-DXB-LON
7. HVB-EK/QF-BNE-SIN-EK/QF-DXB-EK/QF-LON
8. HVB-EK/QF-BNE-SIN-EK/QF-DXB-LON
9. HVB-EK/QF-BNE-SIN-HKT/MNL/OSA/SEL/SGN/SHA/TYO-EK/QF-DXB-
EK/QF-LON
10. HVB-EK/QF-BNE-SIN-HKT/MNL/OSA/SEL/SGN/SHA/TYO-EK/QF-DXB-
LON
You're getting the error because there are no published fares that could be used to price the routing you're after. Neither routings (DXB-HVB, and HVB-LHR) feature MEL in the transfer points.
Also QF only operates BNE-HVB.
Hope this helps.
