Base64ToGallery - Error while saving image - Android

Hello,
I’m creating a chat app at the moment, similar to for example whats app and now I got problems with saving the Images on android with the error message: “Error while saving image”. On iOS all works without any problems.
I’m getting the Image from the Server as a base64 string.
I thought it maybe has to do something with the permissions but I checked them with @ionic-native/androidPermissions and this @ionic-nativ/diagnostics and both plugins told me that I got the required permissions.

The Code of the Saving Image function:

 savePhoto(base64String){
 this.base64ToGallery.base64ToGallery(base64String, {prefix: '_img', mediaScanner: true}).then(
    res => {
        console.log("Saved image to gallery:", res)
       //more code on this point, but shouldn't affect the base64 plugin
     },
    err => console.log('Error saving image to gallery:', err)
    )

Code of grantPermission with androidPermissions:

this.androidPermissions.checkPermission(this.androidPermissions.PERMISSION.WRITE_EXTERNAL_STORAGE).then(
      result => console.log("Has Permission?", result.hasPermission),
      err => this.androidPermissions.requestPermission(this.androidPermissions.PERMISSION.WRITE_EXTERNAL_STORAGE)
    )

Code with ionic diagnostics:

getStatusPermissionDiagnostics(){
    let successCallback = (isAvailable) => { console.log('Is available? ' + isAvailable); }
    let errorCallback = (e) => console.error(e);
    this.diagnostic.getExternalStorageAuthorizationStatus().then(
      successCallback).catch(errorCallback)
  }

grantPermissionDiagnostics(){
    let successCallback = (isAvailable) => { console.log('Is available? ' + isAvailable); }
    let errorCallback = (e) => console.error(e);
    this.diagnostic.requestRuntimePermission(this.diagnostic.permission.WRITE_EXTERNAL_STORAGE).then(successCallback).catch(errorCallback)
  }