I applied for the Maybank's Ikhwan Mastercard credit card using Maybank's online application form and just now received confirmation by email saying my application was unsuccessful.

What surprised me was it took only a few hours from when I submitted the application for them to email me back saying I was rejected. It makes me wonder if I entered something wrongly or something.

Is it possible to call Maybank to ask why my application was unsuccessful?

And is it possible to apply credit card again so soon (even if other banks?)

I don't have Maybank debit card, but I have Bank Rakyat debit card.. and have used at Amazon Japan back in 2013.

Don't think there is any problem. You can try.