Empty Graphs (-nan) while rrd get updated

Hi Folks,
this is a fresh installation where everything looks good beside that there is nothing inside all graphs ;-((

Apaches Error-log shows this, which is of course the wrong path:

script '/opt/observium/html/ajax/graph.php' not found or unable to stat, referer: https://thor-odd.f1-online.net/ajax/entity_popup.php
[Fri Jan 13 08:50:32.122092 2017] [:error] [pid 63712] [client 193.34.29.250:42704] script '/opt/observium/html/ajax/graph.php' not found or unable to stat, referer: https://thor-odd.f1-online.net/ajax/entity_popup.php
[Fri Jan 13 08:50:32.126161 2017] [:error] [pid 63708] [client 193.34.29.250:42706] script '/opt/observium/html/ajax/graph.php' not found or unable to stat, referer: https://thor-odd.f1-online.net/ajax/entity_popup.php

I already tried all possible entries within config.php but still no change?

Thank you,
Wolfgang