Depending on the extension in use, it is possible for a backend authentication system (such as a MySQL database, LDAP directory, etc.) to become unreachable, resulting in an internal failure during authentication attempts. Because of the way such internal failures are handled, this can cause the Guacamole login screen to fail to display entirely, masking any notification that might advise the user of the failure.
The authentication system should fail cleanly. If doing so does not reveal sensitive information about the system, the fact that the system is currently unavailable should be relayed to the user such that they can contact their administrator. If doing so does reveal sensitive information, the login attempt should fail cleanly as if the user entered invalid credentials.