We've recently deployed to new servers and some users have had trouble logging in.  This can be resolved by deleting your browser cookies.  Here are some instructions on how to delete your cookies.

Internet Explorer

In Internet explorer, click “Tools > Internet Options” 

Then on the general tab, under the browsing history section, click delete.

Then follow the instructions in the image below:

Then click Delete, then click OK then try logging in again.  You may need to restart your browser if you still cannot log in.