You have added a second API Appliance to your Enforce Server, and have followed the documentation for configuring your Client.
However, requests for the second Appliance are returning an error:
"Missing request header 'X-SYMC-DLP-DetectorID' for method parameter of type String"
This detail was not required for the first Appliance.
Release : 15.5 GA
Component : DLP API Appliance
There was a defect in the 15.5 GA release, which required this information when more than one Appliance was present.
Update the Appliance to the 15.5 MP1 release.
This patch is available at the main portal for software downloads, under the section marked "DLP15.5MP1Appliance ":
Symantec_DLP_15.5_MP1_Appliance_15.5.1.0.234456.zip
Details confirmed in the 15.5 MP1 Release Notes:
4225918 = The API Detection Appliance erroneously required the HTTP header X-SYMC-DLP-DetectorID