I’ve found a bug introduced to a common bit of code by a recent update to WordPress. It’s not exactly a bug in WordPress, it’s more a change to something a lot of people have been using for some time which breaks what we’re doing.
The result for you may be that while you can click an “add link” button, and it all seems to be working fine, when you click the “insert” button to add that link in to a text box, nothing happens.
I have found a fix – however, finding the time to roll it out immediately to a lot of sites is proving a bit more difficult. I’m working through sites as quickly as I can, alongside routine WordPress and plugin updates, but you might still be having issues. If you are, you can copy and paste your links (remember the http:// part!) directly into the text box – don’t use the button. Save as normal, and things should update.