The full exception response is:
Server's response could not be processed. Name of HTTP response header is too large
This could be caused by a malformed response, or possibly a misconfiguration.
connection: service.name=HTTPS client.address=192.168.XX.X proxy.port=443 client.interface=1:0.1 routing-domain=default
time: 2019-08-23 17:34:25 UTC
DNS lookup was unrestricted
User-Agent: Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/58.0.3029.110 Safari/537.36 Edge/16.16299
user: name="DOMAIN\user_account" realm=AUTH_REALM_NAME_IWA
authentication status='none' authorization status='none'
EXCEPTION(invalid_response): Total length of HTTP response headers exceeded configured limit
Last Error: Unexpected transaction termination on URL(https://mywebsite.maindomain.com/Account/LogOn?ticket=SAMPLE-KJjhx3xJ82kx8cXSKlcs9xs), client IP(192.168.XX.X), server IP(27.4.X.X): Total length of HTTP response headers exceeded configured limit (100000)
url.category: none@Policy;none@YouTube;none@Local;Education@Blue Coat
total categorization time: 7
static categorization time: 7
server.certficate.hostname.category: none@Policy;none@YouTube;none@Local;Education@Blue Coat
total categorization time: 1
static categorization time: 1
DSCP client outbound: 65
DSCP server outbound: 65
The problem here is that the server was sending a response with an individual header name greater than 8kb. This would mean it would not fit in one block. This is hard coded and cannot be changed.
To verify this take a developer tool trace in the browser, save it as a .har file and using application of your choice open it. Look over the request causing the issue which received the 502.