When trying to save the URL from the storage to RealtimeDatabase I get a DatabaseException Error

Question

I am trying to store an image url which I get by uploading it to a FirebaseStorage in a RealtimeDatabase. The former works without any problems. The image is created in the storage. However, in the OnSuccessListener I cannot save the URL in realtime. Then I get the following error:

com.google.firebase.database.DatabaseException: Failed to parse node with class class com.bkoubik.niceplace.AddNewPlace$4$1$1

AddNewPlace.java class:

 StorageReference storageRef = FirebaseStorage.getInstance().getReference();
 StorageReference mountainsRef = storageRef.child("images/"+ UUID.randomUUID() +".jpg");

 Uri uploadUri = Uri.fromFile(new File(imageUri.toString()));

 mountainsRef.putFile(uploadUri).addOnSuccessListener(new 
 OnSuccessListener<UploadTask.TaskSnapshot>() {
        @Override
        public void onSuccess(UploadTask.TaskSnapshot taskSnapshot) {

            bar.setProgress(0);
            status.setText("100 %");

            mountainsRef.getDownloadUrl().addOnSuccessListener(new OnSuccessListener<Uri>() {
                @Override
                public void onSuccess(Uri uri) {

                    DatabaseReference ref = FirebaseDatabase.getInstance().getReference();

                    //UploadUriForPlace up = new UploadUriForPlace(uri.toString());
                    String placeId = ref.push().getKey();

                    ref.child(new Date().toString()).setValue(uri.toString(), new OnSuccessListener() {
                        @Override
                        public void onSuccess(Object o) {
                            Toast.makeText(AddNewPlace.this,"Ist oben, danke!",Toast.LENGTH_LONG);
                        }
                    });
                }
            });


        }
 });

Uri is not empty: