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

[DeBuXer]

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';

[WolfgangFahl]

You might want to try pymediawikidocker to test things.