It gets stuck right after completing all the details of the card. After closing the popup the transaction goes into pending state and after 30 mins it gets cancelled. Sometimes though you can see the bank confirmation screen flickering in the gray popup for a mere second. It seems the popup poorly handles HTML iframes.
Bank: Swedbank LT, Mastercard Debit
Device: Samsung S7, Android 7.0
UPDATE: managed to get through the gray overlay by randomly tapping and swiping in the gray area. The mastercard secure greeting text appeared and then logged in as usual. Seems like your frontend developers got work to do