Problems with webcron

After generating an auth_token and testing the webcron I get this error message:

Error: Entry "Piwik\Plugins\Monolog\Formatter\LineMessageFormatter" cannot be resolved: Parameter $logMessageFormat of __construct() has no value defined or guessable
Full definition:
Object (
    class = Piwik\Plugins\Monolog\Formatter\LineMessageFormatter
    lazy = false
        $logMessageFormat = #UNDEFINED#
        $allowInlineLineBreaks = (default value) true

I can’t find any issues about this error and I have no further ideas how to solve this. I was just following this tutorial (the webcron part):

Matomo is installed in an subfolder and is using https. Tracking works fine.

This is my webcron (anonymized and token removed):[here is the correct token]

Any help is much appreciated!

The monolog plugin was inactive. Do not know why.
Found the solution here: