When content was migrated to UX.SE from UXExchange.com, the numbers uniquely identifying each post changed. That means all links containing a "uxexchange.com" link are broken, and replacing the url with ux.stackexchange.com doesn't fix it. E.g.
http://uxexchange.com/questions/1/ does not map to
https://ux.stackexchange.com/questions/1/, you'd have to find the post of the matching title/content.
It also seems not all content was migrated (or the matching question was deleted). Removing the link is an okay last-resort. A few of these UX Exchange questions are also completely useless on this site, if you see one go ahead and flag requesting deletion; I've seen a couple of those (closed discussion thread where nothing valuable was actually in any of the posts)
If the answer's just a (now dead) link, flag it and it can be deleted
We have 35 posts containing UX Exchange links. Finding the posts they link to is a crap shoot; you can try searching for the user/post title referenced with the link, there's no sure fire way to find these. But I'd like to get these links fixed, or at least as many as possible. Some links without enough content may be impossible to fix...not sure what we should do in that situation.
Please help by tracking down the post originally intended to be linked (as best possible) and editing in that URL.