This is most likely because you are not logged into the right account.

Login to cPanel. Click on the mail icon. Click on add remove accounts. Find the account you would like to log into and type this email account's password.

Next you have the option to choose several webmails. Click the Horde graphic.

You will now see after you login "Welcome"

If it says anything other than the email address you are trying to read,you will need to use the log out button in horde to make sure your browser is completely logged out.

Now open a new browser window and try again. this should clear up any possible problems.