I used a thread about a similar problem that I reset the Extender and needed to re-enter a password (using the app).  It wasn't accepted and read "Failed. Please avoid entering overly long..."  and like was posted in the thread, then used the web-based approach and was able to re-link it to the router and all the lights are working.  The problem now, is that I can't connect my laptop or Android to it.  I can find both the 2.4 and 5 Ghz options and try to enter the password that I just added... and it says the password is "incorrect".  

On the App, the password works!