Symfony - Webpage drops error after installing API-Platfrom

After I made it to install the API-Platform and create an entity with the maker-bundle and also updated Symfony to version 4.4, I get an error when I open my webpage.

And although that I activate the debugging mode, I don't get a proper error message, that's sucks....

In the passt I often getting a more detailed error message when by running:

php bin/console clear:cache

...and yes this time too. The error message said:

In FileLoader.php line 180: 

  There is no builtin action for the collection GET operation. You need to define the controller yourself in . (which is being imported from "<path>\config/routes/api_platform.yaml"). Make sure there is a loader supporting the "api_platform" type.  

In ApiLoader.php line 177:

  There is no builtin action for the collection GET operation. You need to define the controller yourself.  

I don't know where the problem comes from and this time googleing gets also no helpful links...

So and now ???

I remember that I have updating Symfony the dependencies to the Symfony version 4.4. But the api-platform/core was not updated to a new version, it was still on version "2.1"

So I decide to update this and all the other dependencies.

There for in my composer.json I changed the version of the api-platform/core from "2.1" to "^2.1" to let composer decide which version should be installed.

I save the file and run the command:

composer update --with-all-dependencies

As it runs it ends up with an error:

 Could not delete C:\Users\userap\Desktop\_Workspace\WebDevelopment\WebClient_2\dosierung\vendor/sensio/framework-extra-bundle\Resources\config:   
  This can be due to an antivirus or the Windows Search Indexer locking the file while they are analyzed

Okay the last sentence sounds plausible to me. Means that a service from Windows has just access the file in the moment when composer want to do some changes.

So no problem, I run it again and this time it works, perfect.

Composer has updated the api-platform/core to the version "2.6.8"

I open my webpage and...YES the 500 error is gone and my page was loading, lucky again.