Confirmation email no longer arrives

Hi,

I have installed your plugin on an important site for more than a year + I had just installed it on another site with the same request.
The customer registers and an email arrives that the admin must approve.

On site 1 it always worked until the other day. Now the registration confirmation nor the approval confirmation to the admin no longer arrives. It only reaches the customer. Same as on site 2, everything worked perfectly for a month, then the emails to the admin stopped arriving. Is this a bug in the new version?

How can i solve?

Thanks for support
Linda