2.6.15.1 – where's the changelog?

better late than never, i guess.

You are using a beta version, hence the .15.1 at the end. We recommend you to uninstall this version and install the stable one from here: https://downloads.www.ads-software.com/plugin/wysija-newsletters.2.6.15.zip

And here’s the changelog for all other versions: https://www.ads-software.com/plugins/wysija-newsletters/changelog/