SimpleCMS prompt for endless Refresh

Hello,

I've just added a new site with SFTP details (have verified that these work).
But after adding this site, I get a prompt that the site is loading longer than expected and to refresh the page.
I click the Refresh button on the prompt, but am prompted again and again to refresh.

This only happens with one site. All the other sites are fine. The difference with this site is that the SFTP details are pointed to another server. But I have verified that the credentials supplied in the SimpleCMS Add Site page are correct as I am able to use the same details using WinSCP.

Some help would be very much appreciated. Thank you.