Error when creating htpl / stpl ....

There has been an issue reported on recent releases where some API calls are routed out of the central vs calling localhost. I suspect this may be related. should be fixed in next minor update.
In the meantime you could try editing the /etc/hosts file on your central server and add a new entry by replacing IP_CENTRAL with your actual IP address and FQDN with the full name of your central:
 

IP_CENTRAL FQDN

Note that FQDN is to be replaced by the name as you use it in the URL to access your Centreon server.