Hi hojo,
I believe I am familiar with the issue you are describing, although I cannot be entirely sure.
We know that this can happen when the user clicks the UI before it has completedly loaded, which can take a couple of seconds. Instead of getting the payment form displayed to them they see a contact "Loading" spinning wheel.
In every reported instance of this issue I have seen it has occurred because of an implementation that has been slightly modified by the merchant to allow for an "other info" page to display, such as an additional confirmation page or the like, before proceeding to click the "Pay" button (or whatever this button has been named with the implementation).
It occurs because the "Pay" button can be launched before the page has loaded completely. Browser console logs typically show an error such as "The target origin provided (js URL) does not match the recipient window's origin (merchant URL)". This use-case has occurred to a very small degree, to the best of our awareness, which has relied on merchant feedback and/or reports of this occurrence.
If I am correctly assuming that you are seeing what you describe for this known cause, I can advise that our developers are currently working on an enhancement for an upcoming release to address this concern. Although we do not have an ETA for release, we are hopefull it will be soon.
In the meantime, if possible, you may want to add in a message to ask the end-user to allow the page to complete loading before proceeding. Difficult in the impatient world we live in, but there is physically no way to prevent the end-user from clicking on the "Pay" button before the page loads completely currently.
I hope this has been helpful to you.
Thank you,
Elaine