Wordpress - Can't Add Blocks with Gutenberg Editor

I came across an issue with the Gutenberg editor, wherein the blocks interface was visible, but I was unable to add anything (the option to do so was disabled)

As noted by this Wordpress SE answer by emat, the fix was to go to Users -> Profile, and under Personal Options

1. check Disable the visual editor when writing
2. save
3. uncheck Disable the visual editor when writing
4. save

You may only need to do steps 3 and 4; I had to check then uncheck; I presume it sets/resets the appropriate flag in the database.

I believe the cause to be something to do with forcing use of the Gutenberg editor for only Post types of post, but it's possible that there was another cause.

The site in question was, at the time, running Wordpress v6.0.2