In my Dev instance after upgrading to 7.5 from 7.4.3 several Admin pages won’t display, including Studio.
I ran a QRR and some of the other repairs.
It seems to me that the QRR did not go all the way through though, stopping at Repairing Relationships… and does not show the ‘Return to Administration page’ at the bottom.
Running QRR on my other DEV instance has a lot more lines of activity.
Solved it:: I had PHP version 5.3 in place; after upgrading to 5.4, everything works again.
Unfortunately, the package upgrade preflight check didn’t mention that.