On the Add Credit Card form, where you input card details, it sometimes happens that the 'OK' button will not be shown correctly.
If you experience this issue, you'll need to clear the iPayment form cache. This can be done in the following ways:
If you're running a more recent version of iPayment, you can use the 'Clear Cache' menu item in the Add-on menu.
If this does not help, you have to go to the following path and delete all .xml files from the folder; %appdata%\Boyum IT\BOY_E0_MODIFICATIONS\CachedData