I found that it was a problem with the URL generated including spaces and pipe characters at the end. For example:
https://mysite.org/wp-admin/post-new.php?text=&popupurl=http%3A%2F%2Fwww.something.com&popuptitle=Some Title | With Spaces | And Pipes
If you delete the stuff that has the pipes at the end, it seems to work.
https://mysite.org/wp-admin/post-new.php?text=&popupurl=http%3A%2F%2Fwww.something.com&popuptitle=Some Title
