Bumping this thread since we're experiencing something very similar with clients being unable to place any items inside of their cart on our end. Mind you, they're using a very outdated IE9 and lower, so that would explain where the complications are coming from. However, in X-cart 5.1, these users were able to successfully browse the website if we had Aggregate CSS files Aggregate JS files checked on inside of performance. However, ever since we upgraded to 5.2.4, these IE users have not been able to purchase anything of our website.
This wouldn't be so much of a problem, however, a majority of our clients, sadly, use IE9 and lower. Would anyone happen to have any insight on how we can go about fixing this issue? It's going to be a problem with the CSS and JS no doubt, and the comparability settings on said browsers don't work (we've tried).
Edit:
Here's a snapshot from one of our clients of the error message they get while browsing the site. It's also reported that, despite what size their window is, it will always display the website in the mobile view mode.
http://i.imgur.com/CCFsAqw.png