Newsletter not saved in 5.5.1

I see this has been an ongoing issue with 2.0 for a while now. We just started experiencing it after upgrade to WP 5.5. Newsletters are NOT saved anylonger. Possibly issue with the new db structure WP is using. We’ve used the plugin for 10 yrs. now and really hate to having to change to a working one. As is the plugin is not compatible with WP 5.5.1 and this needs to be fixed ASAP //THANKS