I just realised that the reason that I had debt was because on a previous turn I had played Capital. Now I know that after playing the capital card, you can only begin paying debt off once your turn is over, but the incident occurred on a subsequent turn. The debt that I was paying off had previously been accrued and so I no longer had the Capital limitation of only being allowed to pay the debt off at the end of my turn.
Hi, in game 5199253, towards the end (possibly my very last turn), I had 3 coins in play, 3 debt and 1 buy. I chose to pay my 3 debt. Immediately afterwards my turn ended. So if I had wanted to buy a 0-cost card, I couldn't have. I believe that I am correct that paying debt does not count as a buy?
EDIT: Not sure if this counts as an interface issue, but not sure which other forum it should have gone in.
Also still waiting for my confirmation email (it also took me a while to get a confirmation mail for this support forum). I tried several accounts, have constantly checked my spam folder, have retried sending the confirmation mail using the Account tab in the game, etc, etc, etc ...