CTRL+SHIFT+8 and some others not working

I'm trying to find the cause of this problem: CTRL+SHIFT+8 no longer works.
This is the standard key combination in MS Word and another text editor I'm using for the command "view hidden characters" (such as spaces &c).
I can always assign a different key combination to that function; that's not the problem.

I'm just annoyed that it won't longer work. It's obviously not an MS Word problem, since the same thing occurs in the other word processor I'm using. (And vice versa it's not a problem with that word processor.)

When I try to assign key combinations in that other word processor, I notice that there are other CTRL+SHIFT+numeral combinations that it currently doesn't respond to:
CTR++SHIFT no longer works with 1, 3, 8 and 0.
With 2, 4, 5, 6, 7 and 9, it's okay. (I.e. I can a assign a function to it in my word processor, and it will work.)

That's weird. Does anybody know of something in a Windows 10 update that may have caused this?

Other software highjacking a key combination might be a cause too, of course. But I haven't installed anything recently that I'm aware of.
It's not a hardware problem (I've tried another keyboard, same problem).

It's not a huge problem but it's so random and it worries me that I can't find the cause.
Thanks.

Okay thanks. Didn't know that tool.
It does list Ctrl+Shift+, 3 & 8. But it doesn't say what they're registered to. (And nothing seems to happen when I press them.)

The tool turns up a very short list, by the way, compared to the screenshots on that site. Just these three plus:
Hot Key : Alt+Ctrl+Delete
Hot Key : Alt+Ctrl+J (I know what this is from)
Hot Key : Alt+Ctrl+K (Kindle)
Hot Key : Alt+F4

I had a similar problem and it was caused because I had been using shortcuts for macros and had subsequently deleted the macro leaving the shortcut blank.

I wrote a VBA script to look at all the shortcuts by enquiring on KeyBindings(n).Command where for all the values of n up until it stopped reporting.

Then for the shortcuts that were reported blank I deleted the entry using KeyBindings(n).Clear (starting with the largest value of n where I wanted to delete the shortcut and working down to lowest one I wanted to delete because it renumbers the netries every time you remove one

Andy