Does not work under Windows/IIS

WP4 under Windows with IIS has a defect that prevents you to login to secondary sites – it complains about browser cookies. The solution for this is to define('COOKIE_DOMAIN', '');, among other things (see this article).

And this solutions is not compatible with WordPress MU Domain Mapping 0.5.4.3.

https://ww.wp.xz.cn/plugins/wordpress-mu-domain-mapping/