Endpoint Protection client appears offline with no green dot. A HTTP 400 error is observed on the client under Help | Troubleshooting > Server Connection Status
Enabling SECREG (linked article below) debug logging indicates too much Post Registration Data
09/10 11:56:28 [10804:9184] <CHttpReg::ProcessReq> Post Data ContentLength = 11872, Configured MaxPostDataLength = 8096.
09/10 11:56:28 [10804:9184] 4 Too much Post Registration Data.
09/10 11:56:28 [10804:9184] 192.168.1.100--FAILED
SEP 14.2 MP1 and newer
Increase the size of the allowable client registration data, which can be configured through a registry key.
HKEY_LOCAL_MACHINE\SOFTWARE\Wow6432Node\Symantec\Symantec Endpoint Protection\SEPM
Add DWORD MaxPostDataLength with decimal range of 8192 (min) to 131072 (max)
Essentially ((8 – 128) KB * 1024)
For the above example 12 KB * 1024 = 12288 to fit the blocked ContentLength of 11872 as noted in the error message above.
Restart SEPM Services to apply the change.