As of the Phoenix 188.8.131.528 release we have changed the way Derender works. When you derender something now it is added to the assest blacklist. This allows you to permanently derender things.
Do NOT derender any of your alts, or they will not be able to log in correctly.
In layman's terms, the asset blacklist is a shared resource. If you derender an alt, and subsequently try to log in with it, during login, Phoenix sees from the black list that this account should not be displayed, and so it doesn't. You will find the account placed at 10, 10, 10, and likely unable to move.
To Derender an object right click on it → more → more → derender
To rerender it you need to remove it from the blacklist.
The object, or avatar will render in a few seconds.