This is/was (highly likely) caused by the enablement of the Mendix Web Application Firewall (WAF) for the licensed regions of the Mendix Cloud (see our release notes).

Subsequently, this afternoon, some changes to the Mendix Web Application Firewall (WAF) were made to resolve some issues customers were encountering. Are you still encountering issues? 

If you’re still encountering issues, please create a Support Ticket (and then please share the URL you were trying to access).