Nodejs SDK - Download File - access_denied_insufficient_permissions - Access denied

We are using the box nodejs sdk. I cannot download a file via API. Reading a directory / files is working great. When I will use the fileID to download the file to process it in my script I always get:

Error on Download Error: Unexpected API Response [403 Forbidden | 98ye1xgr46e3s3y2.015610c39a9363c40e9d179fae0f64a6c] access_denied_insufficient_permissions - Access denied - insufficient permission

Im using OAuth with the service Account and already have given write access to the folder. The user is also Co-Owner of the directory.

Im trying since hours to figure it out. What am I missing?

Code:
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// Get Client
const sdkConfig = require('./box.config.json');
const sdk = BoxSDK.getPreconfiguredInstance(sdkConfig);
const client = sdk.getAppAuthClient('enterprise', sdkConfig.enterpriseID);

// Working Code for Folder-Contents
client.folders.getItems(folderId, { fields: 'id,type,name,modified_at,modified_by' });

// Download a File with fileId (!! Not working)
client.files.getReadStream(fileId, null, (error, stream) => {
  if (error) {
    console.error('Error on Download', error.toString());
    return;
  }

  // write the file to disk
  const output = fs.createWriteStream(targetPath);
  stream.pipe(output);
});