This isn't a bug in XenForo. You've got a flash blocker installed which has blocked the flash uploader and provided you with a button to click on to unblock it. Before you clicked it you had the basic uploader and after, the flash one was working.
I've tested this in Chrome with
this extension installed and got the same results. I assume it's the same flash blocker you're using as it's got the same icon in the address bar. Clicking on that icon will give you an option to "Always allow flash on this site", clicking that will solve your problem.