Unable to add external link

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I've been using Concrete 5 more than 3 years and haven't had a problem adding external links. Now, for some reason, when I edit the a text box and add a link, then click the 'insert' button, the pop-up editing box just goes to blank again and no link is inserted.

I tried this before and after updating to the latest version, still no luck. Can anyone help? Thanks in advance!