problem with multisites on subdomains

I have installed WordPress as Multisite and I have few blogs on different domains. All work well

However, I have a blog in a domain, lets call it example.com. The blog on example.com works, but then I created a new blog under tech.example.com.

Now the problems is that I simply cannot login to tech.example.com. I can login to example.com. it complains about the cookies.

Any workaround for this issue? (other then to clean the cookies every time).