The plugin in question that I believe is displaying this message is the User Role Editor, although I do not believe it is causing the issue as users who should have permissions to make posts on the WordPress site are listed as having a read-only role and are unable to post. I believe this problem has something to do with caching of Nonces but I cannot prove it. I am not encountering this issue with similar WordPress sites I host. For a while, we could not make posts via the WP-Admin in Chrome but could do so via Internet Explorer but now both do not work. Does anyone have any thoughts?