Thanks for all the input. I just kept trying, and eventually succeeded in getting my trip booked (I think the Mastercard SecureCode check may make things worse as it triggers additional handovers - but then again, this works fine on most other websites).
Regarding OTA, the fares I found were at least 50% more than what A3 had on offer - I ended up booking what I think is a great fare CPH - RHO - CPH, €137.
One additional question: I could only apply my gold upgrades to three of the four segments. For the one where (for now) I'm in Y (A320), do I go for an exit row or row 3?