This worked for us:
edit /etc/httpd/conf.d/ovirt-engine-grafana-proxy.conf
add "ProxyPreserveHost On"
should look like this now:
<IfModule !mod_proxy>
LoadModule proxy_module modules/mod_proxy.so
</IfModule>
<Location /ovirt-engine-grafana>
ProxyPreserveHost On
ProxyPass
http://127.0.0.1:3000 retry=0 disablereuse=On
ProxyPassReverse
http://127.0.0.1:3000/ovirt-engine-grafana
</Location>
systemctl restart httpd