Update on my OP.
Apparently I didn't enable international roaming before leaving Thiland, so my Thai SIM does not find a network connection here in Europe.
When I proceed with the upgrade this evening, and enter my card details, I get asked for the OTP, which I obviously don't receive, since my Thai SIM doesn't have network connection.
I click resend OTP twice, but of course, I still don't receive a code.
When I press the cancel button in that small window where I have to enter the OTP, it gets instantly approved.
Within the next 5 seconds I receive a message in the Kbank app that money has been withdrawn, and get the message from Emirates that my upgrade has been confirmed.
Thai bank security at its best.