do not execute synchronization concurrently

                syncTask.addCallbacks(callbacksPull);

                callbacksPull = new ArrayList<>();

            }

            syncTask.executeOnExecutor(AsyncTask.THREAD_POOL_EXECUTOR);

            syncTask.execute();

        } else if (!onlyLocalChanges) {

            Log.d(getClass().getSimpleName(), "... scheduled");

            syncScheduled = true;