This is a known issue that is currently being dealt with.
When installing Plex from the dashboard, the page will reload with a timeout error on the install. However, manually refreshing the page to the https://my-server-ip shows Plex is installed. Navigating to Plex from the menu works, Plex works, just a header refresh issue.
Additionally, when installing BTSync, the page refreshes informing the user that the package has been installed, however; there is no BTSync menu link and the dial under 'Service Controller' shows installed while the badge says 'Disabled'.
BTSync Package doesn't appear to install at all. (this is a separate installer issue - also being addressed)