I've got the app pinned to the taskbar.
If I click on the app button on the task bar when the app is behind others, DOpus comes to the front & is OK.
However, if I click again on the taskbar button to shrink the app., it crashes & restarts.
This has been happening for some time now, over quite a few updated versions.
Gary
All clicking the taskbar should do in that situation is minimize the window.
What happens if you minimize the window using the top-right buttons?
Minimizing using the top right button has always worked OK.
In fact, that's the way I've been minimizing it for some time .... but every now & then I forget & use the taskbar.
Gary
If Opus is pinned to the taskbar, try unpinning it (and re-pinning it afterwards). That fixes some issues with the taskbar, although this is the first time I've heard of this particular one.
If that doesn't help, are you running any tools which modify or hook standard Windows behaviour, window borders, the taskbar, and so on?
Solved!! 
Have been running an app "4T Tray Minimizer" which adds 4 extra buttons to windows title bars. Have been using one of these to force a window to stay on top.
As soon as I kill this app, problem has gone.
Will have to search for another app to keep a window on top.
Thanks for the help
Gary