Subscribe2 gives an error

Hi, I am using Subscribe2 plugin on a site and in attempt to subscribe, it gives a server error: Sorry, it seems to be an error on the server. Please try again.

I have read other threads about the same issue and the plugin author suggested to use an email address from the same domain to solve this. I tried that too and it still gives me the error. On the other hand, when I used this plugin on a different site and used a different email address from domain, it still worked fine. But not on this particular site that I am working now.

Other things to note: even with this problem I can see the email of the subscriber is entered into the list on the plugin, but no emails notification is sent neither to admin nor the subscriber. The new posts don’t get sent out either even if I enter email addresses manually to the list.

What can the problem be and how to solve it?
I appreciate any insight to this.