Use threading.Thread for @background decorator again.

app/decorators.py

-from . import socketio
 from flask import abort, current_app, request
 from flask_login import current_user
 from functools import wraps
+from threading import Thread
 
 
 def admin_required(f):
@@ -26,8 +26,8 @@ def background(f):
     @wraps(f)
     def wrapped(*args, **kwargs):
         kwargs['app'] = current_app._get_current_object()
-        kwargs['current_user'] = current_user._get_current_object()
-        thread = socketio.start_background_task(f, *args, **kwargs)
+        thread = Thread(target=f, args=args, kwargs=kwargs)
+        thread.start()
         return thread
     return wrapped