Hello, I recently upgraded our 2860ac to the latest firmware. Now when I go to the web configuration pages, I get to log in, click on any of the navigation options and the router crashes and reboots. Any ideas?

Doesn't sound good... corrupted config ? Try backing up, resetting and then seeing if it's consistent. Restore the config and see if that 'breaks it'. At least you then know it's the config and you have to start config from scratch. Obviously if you can't get to the backup page, you can't do it (maybe by TFTP ?) of if someones can tell you the direct URL. Check you're using the right PSU and remove all unncessary connections (USB, ethernet), DSL.