On some browsers users are unable to login. Users also don't get feedback. The Magento system just doesn't respond. Sometimes the login does work in incognito mode.
Some things I already checked
- formkeys are integrated in the forms
"web/cookie/cookie_domain" are for all storeviews set to null. - We run multiple storeviews eg; example.com/en/ , example.com/de/
I also tried modifying the cookie settings in the code as suggested on this url http://blog.chapagain.com.np/magento-admin-login-problem/ (however this article is related to the magento admin, where our problem is the frontend) but also that didn't work.