Yep, it's a known bug.
Three things that have been suggested as workwarounds:
1) Press the OK button and continue with the restart
2) Use the WinX menu (right click Windows Logo on Taskbar) and select a power option from Shutdown or signout
3) create a shortcut to the shudown.exe and place it on your taskbar
a) restart - %windir%\System32\shutdown.exe /r /t 30
b) shutdown -%windir%\System32\shutdown.exe /s /t 30
the /t option is the amount of time to wait before taking action - you can set it to a lower seconds count (/t 15...), set it to 0 or leave off the /t parameter if you want quicker action