Had the exact same issue this week. I would receive a 404 error message when trying to pay, but received a PNR and could access the booking via the app/website. I assumed the first one went through but it disappeared without any warning after a day or so. I checked by chance and couldn't find the booking anymore...

Completely infuriating experience trying to talk to Qatar's customer service agents, who had no idea and kept blaming my bank. I tried two different cards from two different banks/institutions, same problem. Called one bank and as expected, they weren't rejecting the payment.

Finally it worked when I paid via Apple Pay on the app and I was issued an e-ticket shortly after.

But it's left a very sour taste in my mouth, and I'm looking for flights on other airlines to replace this as my faith in Qatar has gone. Really not good enough.