CDSSO : error during redirect to AM login page

Regression on AM-7.0.0-SNAPSHOT
Git SHA! : OpenAM: 7.0.0-SNAPSHOT 2b1a765c973 : WORKING OK
Git SHA! : OpenAM: 7.0.0-SNAPSHOT 8301e3d99fe : DOES NOT WORK PROPERLY - object of this issue

Since this commit, the following error occurs :

HTTP Request: GET http://openam.example.com:8086/openam/oauth2/realms/root/realms/filters_realm/authorize?scope=openid&response_type=id_token&realm=/filters_realm&redirect_uri=http://openig.ext.com:8084/home/cdsso/callback&nonce=9346d44f-d26d-476d-8abe-f49b1663cd04-341&client_id=cdsso_ig_agent&response_mode=form_post&service=igChain "HTTP/1.1 302 Found"

returns this error :

Error 'invalid_request' during authentication because of 'Invalid request on redirect endpoint'

Steps to reproduce with pyforge :

• - git pull PyForge
• - ./cleanup.py -f
• - ./configure.py
• - make sure your /etc/hosts is properly configured for IG functional tests : (cf hosts on docker : https://pyforge.engineering.forgerock.com/docs/getting-started#prepare-docker-on-your-machine)
• - in config.cfg, update the IG section with WEBCONTAINER_TYPE=standalone
• - launch the following commands (on Linux/Mac), in the PyForge root directory :
  export PYFORGE_ROOT_DIR=`pwd`; source PyBot/OpenIG/tools/.qa_tools; ppth
• - launch the tests with the following alias
  rig -s GatewayGuide/CrossDomainSingleSignOn -t When_Not_Authenticated_On_Am_Then_Accessing_Route_Should_Redirect_To_Am -n
  Servers are then available for checks... (NB : test may have been modified, to expect the current error)
• - ./cleanup.py -f