Failure in adding to Astra Main Sidebar

I’m trying to add a left sidebar menu to some pages in my Astra theme on WordPress 5.6.

I’ve tried both the built-in Navigation (vertical) block, and, after installing the Custom Menu Wizard Widget plugin, the Custom Menu Wizard block (both in the new Widgets page to the Main Sidebar).

In both cases, when I try to Save/Update I’m getting “There was an error. [object Object]”

Any idea what’s up? I suspect that it’s a Block Editor issue, but I figured you folks would know more, as it affects you, too.

Thanks in advance for any wisdom…