Joomla webshop cache issues

I've built a website for one of my clients and migrated Mijoshop (Opencart fork) from their old website to the new. It seems to be a stable software, but for a while now we experience a strange issue with the cart/checkout. We get multiple reports from shop visitors that they aren't able to proceed from the cart to the checkout. They hit the 'go to checkout' button and nothing happens. Strange thing is, it happens with random users with random browsers and devices. But it mainly seems to be happening in Chrome. After many tests I eventually experienced the issue myself. I hardcoded the button with an absolute link to the checkout but this doen't work. Then I changed the checkout URL and suddenly I was able to proceed to the checkout. Also when I disable SEF urls, the checkout becomes available. It got me thinking that caching could be the problem. But the Joomla cache is disabled and there are no caching plugins on the server (we even moved to another server). I also installed Regular Labs Cache cleaner to automatically clean cache on intervals, but without success. Only option left is that MijoShop has somekind of cache function, but I can't find anything about this. Unfortunately the MijoShop developers don't answer to my questions, I've tried to contact them several times. I also tried the Opencart forum buth without any luck. So freelancer.com is really my last resort in finding a solution for this problem... There is no caching activated on server level. The issue also happens on other servers. I've recorded a video to show what happens. Maybe this can help: [login to view URL]