When you try to enable the foreman-smart-proxy-plugin with api user and password on the foreman server, the connection via https get a bad request and they try to connect again.
(Problem occurs in Icinga 2 version 2.11 and later)
It seems to be a bug in Icinga 2.
Little work around:
If u use the certificates instead of user and password, it works.
I have created an issue on icinga 2: Icinga/icinga2#8213