APM - Empty payload received for a portal access ressource

For future engineers who might see this question, here the solution from support team (thanks Ruslan):

The BIG-IP system receives unexpected HTTP data from the server. A typical case occurs when a server sends an HTTP response with a Content-Length header value that is smaller than the actual size of the content.

"Extra content in the same packet used to be tolerated by BIG-IP by default. In 12.1.0, ID526760 caused the default behaviour to change so that extra data is not tolerated. A DB variable allows the behaviour to be changed - Tmm.HTTP.passthru.invalid_content_length."

To fix it (workaroud): tmsh modify sys db tmm.http.passthru.invalid_content_length value enable

https://support.f5.com/csp/article/K13223