Hi,
Please understand that we also sleep during the night, and we get many questions everyday. I only saw your message 2 hours ago. Then, I had to check the situation on your website, try to reproduce it on my end, do some debugging, create a patch, test the patch, generate new HikaShop packages with the patch and then write this message. All of this takes time.
The issue comes from modifications we made in the 5.0.0 in order to make the checkout more seamless for customers in some cases. This had the side effect of not checking that the payment method is selected when the "payment" block is in "read only" mode. We didn't expect this to be a thing as normally this setting is turned off, and only turned on when having a "recap" step at the end of the checkout (and by that point, the payment method selection has already been made by another "payment" block with the read only setting off).
I've made a patch in HikaShop so that even in read only mode, the check of the payment method selection is still done at the end of the checkout.
Download again the install package of HikaShop 5 on our website and install it on yours and you'll get the patch.
Another easy temporary solution is to turn off the "read only" setting of the payment block in your Checkout workflow.