- It is possible to give the same shortcut to different functions - e.g. Ctrl+R for Reload and and Toggle Reference mode
- When I added a Shortcut for Reference Mode, Ctrl+Shift+R, and clicked OK I got this error:

And now I sometimes get that error whenever I click OK for a shortcut - even if I don't change it, and even if there's no shortcut for that particular item. I can't fathom an obvious pattern behind the 'sometimes'.

I don't really want to restore Defaults because I'd lose earlier adjustments, and I don't have a list of them. The entries in viewer-webengine.json appears to be incomplete, I have a suspicion that some of my earlier customisations were overwritten by an update. I don't recall ever deleting that file or restoring defaults.
It's a pity that the UI for the Viewer's Keyboard Shortcuts can't be the same as that used in Library Manager and Book Editor.
BR