403 error during admin saves – only when this plugin is active

Your plugin is causing some admin pages to have trouble saving their data. The most consistent one is Appearance->Menus – clicking “Save Menu” results in a 403 error (with a mysterious message from WordPress: “The link you followed has expired.”). It was intermittent on other pages as well, but I didn’t keep notes.

Apparently there is something unique about my site, since no one else has this complaint. But it’s not a conflict with the theme or other plugins – if I change to the 2020 theme and deactivate all other plugins except yours, it still throws the error. If I run my theme and all plugins except yours, it works fine.