@Mike Kadow and I are both having this issue.
When editing a post and trying to insert a link, I use the Internal link type to link to a different post. I am able to search for the page and get the page name to show up in the dialog box, but then the "ok" button is non-responsive. I can cancel properly. If I change the link type to URL and paste in the full URL of the page, I can click "ok" and it will save the link.