Then I de-installed Tweaks.
Did you reboot after that? Tweaks can patch some firmware scripts, and sometimes it will point to a Tweaks script.
Normally the patch is undone when you stop/disable Tweaks (I don't guarantee that's true for all patches), but I have seen in the past that a package is not (always?) stopped before uninstalling, which means that after deinstalling some firmware scripts may fail because the Tweaks scripts are gone.
A reboot resolves that, because all patched files are volatile.
Having said that, I wouldn't know which Tweak could block the GUI shutdown.
I get "Internal Server Error" when starting the Tweak web link.
Again, did you reboot? The Tweaks webinterface is a CGI script. You get an 'Internal Server Error' when the CGI script errors out, which can be the case when parts of the 'old' Tweaks are still running.