Win7 64Bit, nvidia 450 GTS - a little below minimum requiremenets - , UE4 editor built from sources.
Minimizing the editor to taskbar (and leaving there for a while) causes sometimes the editor to spin the cpu up and the application not responding anymore. The gui cannot be recovered from taskbar anymore and UE4Editor have to be killed in Task Manager to get rid of it.
The editor’s log viewer however can be reopened to visible again but not responding to any user inputs anymore.
My windows installation have no such gui issues, but only with UE4Editor.
Unfortunately, it does look like your graphics card is below the minimum recommended specs, which is probably what is causing this issue. When the editor is minimized then re-opened it is putting too much strain on the card and causing it problems. You will need to upgrade your card to be able to run the editor more efficiently. Thank you and have a great day!