Major issue with saving settings

Hi, we are using the latest version of the newsletter plugin, WP 4.8.1 running on PHP 7.0. We have a huge problem that I have never seen before. Both the newsletter dedicated page and the widget throw many errors like:

Warning: Illegal string offset 'name_status' in /wp-content/plugins/newsletter/subscription/subscription.php on line 1371

We activated Twenty15 and deactivated ALL plugins, but the newsletter: Still, no luck. Then I found that I can't save some of the settings. After reloading, the values in the input fields are gone. This might explain the warnings. I checked it in several browsers logged in or logged out. No luck, still the same issue.

I deleted and re-installed the plugin. No luck. After activating I get an "header already sent" error message, even if with no other active plugin. What's going on? Any help would be appreciated.