SEP 12.1 RU6 has been released and now available to download at flexnet.
It has one following as well, make sure you are no longer affected by same.
Errors display after upgrading SEPM to 12.1 RU5
Fix ID: 3685209
Symptom: After an upgrade to 12.1 RU5, attempts to log on to SEPM through the hostname may result in a failure, with the error message “Request contents are invalid” and the error code 0x120c0000.
Solution: Modified tamper detection functionality to handle all host names correctly.
Referene: New fixes in Symantec Endpoint Protection 12.1.6
http://www.symantec.com/docs/TECH230558
Installation and Administration Guide:
http://www.symantec.com/docs/DOC8645