Are you syncing accounts by enabling the ‘Create or Sync Discourse Users on Login’ option? All it is doing is sending user data from WordPress to Discourse. The only field it updates on WordPress is the Discourse Username field. This field is only used by the WP Discourse plugin, so it shouldn’t be causing a problem.
I think that by default WordPress sets the nickname to the same value as the username field. Do you have the iThemes Security plugin installed on your site? The only reference I can find to the error in your screenshot is for sites that are using that plugin.