Follow-up ticket to comments at https://github.com/piwik/piwik/issues/4577#issuecomment-349647360
With Piwik v3.2.1 IP restrictions for viewing dashboards were introduced (https://piwik.org/faq/how-to/faq_25543/)
An unwanted sideffect from this feature is that e.g. the Piwik plugin for WordPress is broken, resulting in unaccessable backend for all users where have the permission to "show statistics" and "Dashboard graph".
I guess it would be best to add a new option which allows to restrict access to the login form only, as this would not break the Piwik WordPress plugin for other (WordPress) users whose IP is not whitelisted.