Thank you for the follow up, @asmecher.
I am on 3.1.1.4. Installed by downloading the zip file, unzipping it in the Generic folder, renaming by deleting “-master”, so it appears on the Plugins page, and finally clicking on “enable”. So, now it shows, but going to plugin settings gives me this error.
I had a similar error before: Can't add item "Custom Page" to Navigation Menu Items in 3.1.1.4, and changing disable_path_info Off and On and restarting Apache each time worked, but not this time.