Subsites dont load - ask for cookies and keep refreshing loop

I'm having issues with my subsites on my multisite install.
The main site is working properly but some subsites are not loading and looks like the browser is keep refreshing and looping the loading process.

I tried deactivating plugins to see what's going on and only deactivating the domain mapping plugin makes the subsites load again.

UPDATE -
Now I get from some subsites Admins (with mapped domains) the news that when they tried to login their browsers ask them about cookies and when clicked ok, they get redirected to the main site.

Any advice?