AttributeError: 'NoneType' object has no attribute 'generate'

I'm trying upgrading our Mediawiki installation from an old version to 1.35 LTS. GraphViz isn't maintained anymore, so I'm now using the plugin Diagrams. The Python application is running and I can draw diagrams directly from the build-in webgui. Only drawing a diagram from our Mediawiki installation doesn't work. Diagrams show the following error when I try to draw a diagram from Mediawiki:

[2022-02-08 13:28:14,315] ERROR in app: Exception on /render [POST]
Traceback (most recent call last):
  File "/usr/local/lib/python3.8/dist-packages/flask/app.py", line 2073, in wsgi_app
    response = self.full_dispatch_request()
  File "/usr/local/lib/python3.8/dist-packages/flask/app.py", line 1518, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/usr/local/lib/python3.8/dist-packages/flask/app.py", line 1516, in full_dispatch_request
    rv = self.dispatch_request()
  File "/usr/local/lib/python3.8/dist-packages/flask/app.py", line 1502, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "dgs/webserver.py", line 68, in renderForWikiExtension
    result=gen.generate('dot',source,targetFormat)
AttributeError: 'NoneType' object has no attribute 'generate'

The diagram I tried to draw:

<graphviz>
   digraph G {
     rankdir = TD;
     edge [arrowhead=none];
     node [shape=box, color=lightblue2, style=filled];
     "dummy1"; "dummy2"; "dummy3";

     "dummy1" -> "dummy3" [label="100Mbps" fontsize="10"];
     "dummy1" -> "dummy3" [label="100Mbps" fontsize="10"];
     "dummy2" -> "dummy3" [label="100Mbps" fontsize="10"];
     "dummy2" -> "dummy3" [label="100Mbps" fontsize="10"];
   }
</graphviz>

LocalSettings.php:

wfLoadExtension('Diagrams');
$wgDiagramsServiceUrl = 'http://127.0.0.1:5003';

BITPlan / diagrams

AttributeError: 'NoneType' object has no attribute 'generate' #13