After confirmation sometimes payment fails

-- HikaShop version -- : 3.2.0
-- Joomla version -- : 3.8.0
-- PHP version -- : 7.1
-- Browser(s) name and version -- : all
-- Error-message(debug-mod must be tuned on) -- : None

For quite some time I am having an issue with a small selection of my orders. I use a plugin for the dutch payment provider Mollie, but to rule out the chance that the issue is related to this specific plugin I temporarily switched to another plugin, but the isse remains: Most orders are processed fine, but around 5 - 10% gives issues:

After confrmation, the user should be sent to the payment provider, but in some cases, the cart is simply empties and the user is returned to the default shop. Some users retry and again, then sometimes it works, sometimes it doesn't.

I have disabled all cache on my site, so this should not be an issue, though the nature of the issue suggest something like this.

Do you have any suggestion what could be the problem?

Thanks. Unfortunately the issue is very hard to reproduce, but I'll try to work on it using your steps.