I try to connect in the backend of our shop with Magento Connect Manager. But it not works. It tells me
But with this user and password I can login to magento.
You need to be using the login information from the Magento.com.....where you login to the Extensions and manage you account on Magento.
I'm having the same issue. I'm using the right username and password.
See my post:
https://community.magento.com/t5/Admin-Configuration-Questions/Magento-Connect-Manager-Invalid-Login...