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 or to be fatally misconfigured, 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. As long as doing so does not reveal sensitive information about the system, the fact that an error has occurred should be relayed to the user such that they can contact their administrator or check the relevant logs.