Going to legacy Plugin Manager from General Config displays token error

Going to the Plugin Manager ([Confluence Address]/admin/plugins.action) from the Plugin Administration Page link on Confluence Admin > General Config displays an error:

The following error(s) occurred:
Your request could not be processed because a required security token was not present in the request. You may need to re-submit the form or reload the page.

This is failing silently / no error/s appear in the Confluence log.