There are 2 possible options to fix this issue:
First is to disable Outlook Anywhere feature, but then you will loose the ability to use your Outlook account outside of your corporate network. Anyway, to disable Outlook Anywhere you should go File → Account Settings... → (select your account) → Change... → More Settings... → Select Connection tab and uncheck the Connect to Microsoft Exchange using HTTP option → Apply.
Another option is to choose Basic Authentification for the Exchange Proxy. Here is how to do this: go File → Account Settings... → (select your account) → Change... → More Settings... → Select Connection tab → Exchange Proxy Settings... → and in Proxy authentification settings choose Basic Authentification → Ok → Apply.