I have always had an issue on my forum with any action that would pop up a confirmation screen (ie, clicking on subscribe to a topic); that it won't show fully - only the pop-up header. If you right-click and open it in a new window, it works and you see the full message about you successfully subscribing - but not if you just normal click on it. It tries to show up, but you only get partial. It's not a browser issue; all users get this. It's not a cache issue - I've cleared board cache and it's not a local cache issue. It has persisted across versions of phpbb, but if I use the default skin, it works fine, so it has to do with this style.
It's especially problematic if the pop-up wants you to confirm something since you can't see that it does unless you open the link in a new tab.