For the CX site, there is currently a "golden" rule: When you change parameters like single/return/Multi-city, change destination/route, always restart from scratch. Clear the browser cookies and restart the browser. Then straight to the stuff you want. From there, it (sorry) might work. Do different and expect the booking to crash somewhere along the line.

Also keep in mind, that, especially on complicated itineraries, it could be somebody snags your seat/booking class on one of the segments, so, once you are there to select (or even pay), the intermediate/final check fails and you have to start again. I also have my doubts, whether the intermediate and final checks are the same. My impression is, the final checks are more thorough/restricting, with the consequence, the booking fails before you can make your payment. It all has to do with a more and more "real-time" booking process, where availability is no longer locked, until the final check, just before the payment. It does have its benefits, though also the consequences. As such my advise to skip things like additional services, you can easily add later on.

Good luck.