There is a confirmed new bug with Google Webmaster Tools where the dashboard wont load for some webmasters.

If you are an SEO or webmaster with a large number of sites in your Google Webmaster Tools account, the sites might not load.

Google said:

I am not sure what number of verified sites you have to reach to hit this bug. I have some accounts tons of verified sites and the dashboard works for me.

Google did share a work around:

Forum discussion at Google Webmaster Help.

The work around