Zextras 2.2.0 unable to install

Hello,

tried to install latest zextras 2.2.0 to Zimbra NE 8.6.0 patch 4 and get the following:

Deploying zimlet ...

[] INFO: Deploying Zimlet com_zextras_zextras in LDAP.
[] INFO: Installing Zimlet com_zextras_zextras on this host.
[] INFO: Upgrading Zimlet com_zextras_zextras to 2.2.0
[] INFO: Enabling Zimlet com_zextras_zextras
ERROR: service.PROXY_ERROR (error while proxying request to target
server: HTTP/1.1 503 Service Unavailable)
ERROR: service.PROXY_ERROR (error while proxying request to target
server: HTTP/1.1 503 Service Unavailable)

unable to install latest version and revert to previous one 2.0.4 which installs OK

Please advise steps to resolve this issue and complete installation of 2.2.0 Zextras