Symptom:
HTTP Status 400 - Invalid direct reference to form login page

Conditions:
1) open the https://cucmip/ccmadmin or https://cumip/ccmservice 

2) Make sure Call manager Ip you are mentioning in the URL is the first time you are typing on the IE/Firefox browsers.

3) On Login page,Type Invalid Username & Password & hit Enter

4) On hitting the Enter,It Errors out as expected -
          Log on failed - Invalid User ID or Password

5) From step 4,Now type the Correct Username & Password on the CUCM admin/Serviceability Login page,
   It shows up the Error "HTTP Status 400 - Invalid direct reference to form login page" even after giving the correct credentials

Note - Issue is seen,Only when the CUCM Ip & Browser cache doesnt have CUCM Ip's details