multicity itinerary cheaper than return
 

When booking the same flights through the multicity searhc, the result is way cheaper than booking the flights as standard return, for example:
AMS-BKK
BKK-AMS costs €1,000

The same flights booked as:
AMS-IST
IST-BKK
BKK-IST
IST-AMS costs €700

Those are the exact same flights and since this is booked in one reservation, all fllights will have the same ticket number and booking reference.

What is the catch?

Sounds like a married segment issue - ticketing will probably fail upon inventory revalidation.

OP, did you manage to ticket it for EUR 700?

It won't fail... Always I book my THY long haul flights like that 😉

HaHa, this TK's IT, what can we expect?

No - when selecting the flights and wanting to continue, I receive an error message 'the selected fare is not available'

Call the call center and tell them the exact flight flights and price, they probably can manage it for you. That's how I booked my last long haul flight. At my last flight, from the Homepage I also got the error, but with Callcenter it worked out.

STR - IST
IST - HAN
SGN - IST
IST - STR

750 Euros