I'm using RH 10. I installed a trial version of RH 2015 that has since expired. Not sure if that would be an issue. I'm not having any other issues with RH. Also, while I still definitely prefer using the RH CTRL+W preview window, the Multiscreen Preview window works from the View menu. I can't see as much at one time, but I can see how it will display and that's what I need.
Hokay, drawing from memory on this as I don't have RoboHelp 10 in front of me. (This computer only has 2015 Release)
I think if you right-click the toolbars you should end up with a dialog that has a few tabs that allow you to customize the icons, menus and Shortcuts. Hopefully the Shortcuts panel looks similar to the one in 2015. Here's what mine looks like:

Maybe find that and perform the reset, then see if things begin working?
Cheers.. Rick 