FWIW, updating via WP continued to generate the error. I had to delete the plugin altogether, re-download it and manually move it into the plugins directory to work.
I’m still encountering the validation error (no client-side validation and the post appears to save but not changes are saved–gutenberg is NOT enabled) but I’ll submit a new post if I can’t figure it out. Thanks.
Any update on the error pasted in my original post?
The validation bug is in 5.8.9 and it manifests when a field fails validation. No feedback about the errant field is provided to the user and one is not prevented from submitting the page. So it looks like “Page updated” but it hasn’t.
That’s really not here or there for this issue however. The 5.8.9 bug was simply the motivation for me to update to 5.9.5, which upon attempting to save any changes, produces the Uncaught Error above.
HTH, thanks.