ArcGIS Server Manager Authentication Error

[JulianoZaban1]

Hello!

We are experiencing a weird problem with our ArcGIS Server 10.3.1. 

Without any kind of interference in server's config or something like this we cannot loggin on ArcGIS Server Manager anymore, by web interface or ArcCatalog. We receive the message "Username or password incorrect".

Looking deeper at Google Chrome Developer Tools > Network I discover the URL called by login process:

"https://<server_name>/arcgis/admin/generateToken"

Accessing this URL directly I get the message: 

Translating the message from pt-br: "premature end of file" (something like this).

Any ideas about this?

Thanks in advance!