I had this problem and there seems to be no easy answer – until I tried a different browser (!!)
In Firefox, the LINK button does not look the same as it does in CHROME!! In Google Chrome’s web browser you can check a box under the link title and the link will OPEN IN A NEW WINDOW! 🙂 That checkbox is not visible in Firefox.
If you’re running the latest version of bbPress (right now it’s Version 2.5.4), that should work!
Whenever that’s happened to me it’s because I’ve had the page open too long, or opened in another window (often on another computer). The page changed elsewhere and I’m trying to make changes to an old draft. Not sure that’s the case here, but it did just happen yesterday for that very reason. Refreshing the page and then making edits always fixes for me.