"Download Sources" failed, can't see source code of python plugin

I am developing a PyCharm plugin, when I look at the sources for the python plugin, I see a bunch of decompiled class files.
 

public Ref<PyType> getReferenceType(@NotNull PsiElement referenceTarget, @NotNull TypeEvalContext context, @Nullable PsiElement anchor) {
    if (referenceTarget == null) {
        $$$reportNull$$$0(2);
    }

    if (context == null) {
        $$$reportNull$$$0(3);
    }

    return null;
}

I do see a banner:


But when I click “Download Sources”, it fails with:
 

4:54:11 pm: Executing 'ijDownloadSourcesabc92b3c-779'...
Calculating task graph as no cached configuration is available for tasks: ijDownloadSourcesabc92b3c-779
FAILURE: Build failed with an exception.
* What went wrong:
Configuration cache state could not be cached: field `__collectionProvider__` of task `:ijDownloadSourcesabc92b3c-779` of type `IjDownloadTask`: error writing value of type 'org.gradle.api.internal.provider.DefaultProperty'
> Could not resolve all files for configuration ':downloadSources_cc00b331-d50f-4fea-9f88-9e9ff07cf2e7'.
  > Could not find unzipped.com.jetbrains.plugins:python-ce:pycharmPC-PC-223.8836.43-withSources.
    Searched in the following locations:
      - https://repo.maven.apache.org/maven2/unzipped/com/jetbrains/plugins/python-ce/pycharmPC-PC-223.8836.43-withSources/python-ce-pycharmPC-PC-223.8836.43-withSources.pom
    Required by:
        project :
* Try:
> If the artifact you are trying to retrieve can be found in the repository but without metadata in 'Maven POM' format, you need to adjust the 'metadataSources { ... }' of the repository declaration.
> Run with --stacktrace option to get the stack trace.
> Run with --info or --debug option to get more log output.
> Run with --scan to get full insights.
> Get more help at https://help.gradle.org.
BUILD FAILED in 793ms
Configuration cache entry discarded due to serialization error.
4:54:12 pm: Execution finished 'ijDownloadSourcesabc92b3c-779'.
```