The Editor has encountered an unexpected error

I often (but not always) get this error when I go into the site editor.
The only other reference to it that I can find on here was caused by a maps plugin, but I don’t see any reference to a plugin in this log report:

p@https://c0.wp.com/c/5.9/wp-includes/js/dist/components.min.js:37:103330
@https://c0.wp.com/c/5.9/wp-includes/js/dist/components.min.js:37:105847
e@https://c0.wp.com/c/5.9/wp-includes/js/dist/components.min.js:37:106206
@https://c0.wp.com/c/5.9/wp-includes/js/dist/components.min.js:37:107495
Bj@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:144:42
Qj@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:176:363
Qj@[native code]
unstable_runWithPriority@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react.min.js:24:27
eb@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:170:165
gf@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:162:319
gf@[native code]
@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:73:231
unstable_runWithPriority@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react.min.js:24:27
$g@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:73:178
ja@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:73:113
@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:240:212
Ki@https://c0.wp.com/c/5.9/wp-includes/js/dist/vendor/react-dom.min.js:44:407
Ki@[native code]

When I get the “unexpected error” message, “Attempt Recovery” always seems to work, so it wouldn’t appear to be too desperate, but I prefer things to be working properly, of course!

I noticed recently that as the editor loads, there is a narrow vertical shape with a dotted outline. It disappears before loading is complete, but it doesn’t look right…
Any constructive help welcome!
Andy

The page I need help with: [log in to see the link]