[Plugin: Newsletter subscription optin module] Double opt-in troubles

I’ve not been able to get the email opt-in to work so far. I have Send Blaster installed and the WP opt-in plugin widget on the site (see here: https://www.blackouteffectors.com/?page_id=206 ).

Mail account seems to be setup correctly and if I turn off the double opt-in setting Sendblaster can import new test addresses. But with the double opt-in on I receive the email with the opt-in link, but after clicking the link it doesn’t confirm the opt-in. It just takes me back to my website.

Do I need to install a PHP plugin even if I’m just using the NSOM plugin widget in a sidebar? I thought that was just to add the plugin to a post/page.

Any help appreciated.

https://www.ads-software.com/extend/plugins/newsletter-subscription-optin-module/