404 error on footer of https://derivationmap.net/user_documentation

[bhpayne]

The page https://derivationmap.net/user_documentation works fine, but there's a "flash" message at the bottom of the page.

 error 404404 Not Found: The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.

The file flask_critical_and_error_and_warning_and_info_and_debug.log contains the message

 2021-01-04 13:43:27,929|controller.py|INFO |696 |page_not_found      |[trace] page_not_found
 2021-01-04 13:43:27,929|controller.py|DEBUG|697 |page_not_found      |404 Not Found: The requested URL was not found on the server. If you entered the URL manually please check your spelling and try again.

That corresponds to the function

 @app.errorhandler(404)
 def page_not_found(er):

in controller.py

Error appears to be isolated to the user_documentation page