WP Multisite: login loop to network admin // main site links redirect all to homepage

I have set up a multisite installation and this is the first time I have this loop issue where i am not able to access the network admin area.

I can access the admin area of each site though.
On the main site (ID 1) all the links redirect to the home. so domain.com/page/ goes to domain.com, no error message.
The other sites work fine, site ID 2 works perfectly, no strange redirects etc. It seems an issue of the main site only.

I was investigating several hours already, tried all the tricks of deleting cookies etc.
Now I'm frustrated and temporarily removed the multisite function so at least the main site works.

All this happend after the domain was updated (the site has been installed using the ip address as domain, and later i updated the data base entries and everything with the domain name. double and tripple checked that already.

I hope you can help me out?