CheckMK Multisite requires slave login before pnp4nagios graphs can be seen on master

Hello, can somebody please help me with the below.

Master: raw 1.5.0p24 on ubuntu 18.04LTS, Sitename omdmaster; 0 hosts 0 services
Slave: raw 1.5.0p24 on ubuntu 18.04LTS, Sitename runkel200; 1 host 21 services

Multisite master slave setup does not allow hovering a slave graph on the master.
Looks like a rewrite loop / constantly refreshes.
Problem goes away for some time if i browse to http://omdmaster/runkel200
and log on a second time here; as long as the browser session exists the master can
view all graphs then.

Both systems have been setup from scratch
according to https://checkmk.de/cms_distributed_monitoring.html
without any further configuration.

Master multisite_proxy.conf:

<Location /runkel200>
    Options +FollowSymLinks
    RewriteEngine On
    RewriteRule ^/.+/runkel200/(.*) http://192.168.200.27:80/runkel200/$1 [P]
</Location>

Here is a screenshot of the siteconfig @ the master:

I tried the below changes, none of them having any impact:

  • Change site configuration to not use persistent connection
  • Change mutisite_proxy.conf to not contain port 80
  • Change mutisite_proxy.conf and site config to use hostname instead of ip

Why is the slave unable to derive the credentials from the master ?
Please note that we have plain http not https (for now) here.