Yes, it seems it is an SSL-missing issue, I am also getting the following error message, in 'Security' page, under 'HTTPS options' section.
Warning! HTTPS/SSL check failed. Please make sure that HTTPS is configured properly.
Actually, I have setup the upgraded store on the new server with an IP address, with a copy of x-payments in a separate hosting account, with it's own SSL, while the old-version current store runs live in the old server.
In order to check x-payments connectivity in the upgraded store, before closing the current live old-version store, an SSL must also exist in the upgraded store environment, but this SSL currently is used for the live site (the old version).
After the current domain points to the new server, the SSL also will be transferred to this server, so I guess, only then, I will be able to check x-payments in the upgraded store.
And before this, I will close the old store and run a final database synchronization in the new server (for any recently-inserted customers, orders, etc) and also move over the final x-payments database to the x-payments new hosting account and eventually point the domain to the new server, transferring also the SSL. So, while the old store is closed and before opening the upgraded store, only then, I can test x-payments.
Is there any alternative procedure so I can test x-payments before closing the old live store and without installing an additional SSL in the upgraded store environment?