Error Message
To access HTML5 GRE Web plug-in from VMware vSphere HTML5 Web client ,
Cell Manager is selected, the User name and Password are specified correctly ,
but "ERROR - Failed to retrieve settings" is displayed then any backups can not be listed.

Cause
This would be happened when Name Resolution is not consistent .

Ex.) Mount Proxy is Windows system
Computer Name : SHORTNAME
Full Computer Name : SHORTNAME
"nslookup SHORTNAME" returns FQDN
"nslookup <IP Address>" returns FQDN
"nslookup FQDN" returns FQDN
cell_info file includes the Mount Proxy with SHORTNAME

Workaround / Fix
The error can be avoided by importing the Mount Proxy with FQDN as Virtual Host .