1. You need to add CRM URL to your Trusted Sites:
2. IIS does not authenticate you as the user that you think it should be authenticating you as.
This one can happen if for example one of your co-workers uses your machine and tries to access the CRM entering their domain login information, then later on their access to CRM is revoked, yet Internet Explorer has already cached their credentials and is trying to use it when you are logged in. The problem is that IIS does not show what Active Directory User it is trying to authenticate you as if you have the option checked to automatically verify your credentials on each visit. The quickest way to fix this is to:
- Do everything in item 1 above.
- With "Trusted Sites" icon selected, click on "Custom Level" button.
- Scroll all the way down to the "User Authentication" section, then check "Prompt for user name and password", clicking on OK.
- Click "OK" again to save settings and exit.
- Close Internet Explorer and Re-open, clearing all cache.
3. The Active Directory domain user you are logged in as does not have a CRM Account created for it.
Simply log in as the CRM Administrator, go under "Settings > Administration > User" and add the user to CRM.