Unable to install El Capitan Update 10.11.3 (Stuck in grey screen)

I have problems to install the El Capitan update 10.11.3. Everytime I hit restart the Mac tries to shutdown and gets stuck in a dark grey screen. The mouse cursor is visible and movable. Special Keys (sound, brightness) and overlay seem to work.

What I have tried so far:

First time I waited like half an hour, then restart with power button and normal startup.

I tried to fix rights with command: sudo /usr/libexec/repair_packages --repair --standard-pkgs --volume /

Same problem.

I disconnected all USB devices.

Same problem.

I tried to update from safe mode.

Same problem.

So what to do next?!?

Are there any protocols and where to find them? Maybe the update file is corrupt, so how can I redownload it again? Any other ideas?

It's an iMac Mid 2011, 27", Timemachine Backups are available.