The card number entered is not an accepted card type.

[Eugene]

Did anyone come across any problem?

The card number entered is not an accepted card type.

I tested different types of cards, and all failed. Usually I should wait for a few days, and the problem will be sorted out automatically.......... 

[ZakMcKracken]

That O2 web page has come controls (in Javascript) to manage the typing, they break copy/paste from Chrome card secure vault and they prevent copy/paste in general (I was trying to copy my card number from my KeePass password manager).

O2 should remove that useless client checks.

[pgn]

They also break the "hold your card up to the camera" method of entering your card details, @Martin-O2 and @EmilieT and @Marjo - had to dig out the laptop to put a tenner on my daughter's phone yesterday - even a switched phone from WiFi to O2 4G to see if it helped... Very off-putting!

[MI5]

There are also redirects when paying your bill online in your MyO2 which all browsers block by default, resulting in a failed payment to O2 but has the money held by your bank for days "just in case".
The whole website is backwards and outdated.

[jonsie]

But O2 will tell you there's nothing wrong with the website or their outdated systems. We've been saying it for years but it certainy seems a little more broken now.