I hit a weird issue today, when I logged into a SharePoint 2013 environment with credentials that weren’t allowed to use it.
The result was an error page and in the url, I could see an AccessDenied.aspx. The thing is, it didn’t allow me to try different credentials.
I found that I have to do put the following on the end of the accessdenied.aspx url –> ?loginasanotheruser=true
That resulted in a new login popup.