After installing Newsletter plugin I am having this error.
Simplest way to trigger:
go to subscription -> Form fields and layouts -> extra profile fields -> add one and click save
Changes are not saved, I'm redirected out of admin panel. Any reattemps to open http://www.mysite.net/wp-admin are giving ERR_TOO_MANY_REDIRECTS error.
If I clear all cookies in my browser (chrome, firefox) I am able to open admin panel, but repating the procedure causes the same effect.
The same effect can by caused by other scenarios while using newsletter admin panel, but this is the one that works always.
Tested on two independent sites (but on the same hosting service).
Thank you for keeping awesome work.