You are most likely running into issues regarding the Internet Explorer Driver and it's faulties.
The driver cannot properly detect if page loading is going on or not. It often fails.
So in your case it might be that selenium logs into the page and while the page that appears after login is being loaded, selenium tries locating the element.
But since the page is still under construction, the element cannot be found.
The solution is to use explicit waits or maybe a sleep.
Another possibility is that you ran into xpath issues. Its not a consistent and guaranteed notation. Browsers might imlement it in a different way.
Always us xpath as last resort. In your case, maybe the classname is unique and you can use By.className instead?