ValueError: Circular reference detected

[lukemyth]

When I run a flask code, it worked well at the beginning, but one day later it always raised an error. The error message is as follows:

[2022-09-08 10:50:36,904] ERROR in app: Exception on /sim [POST]
Traceback (most recent call last):
  File "/root/miniconda3/envs/simple/lib/python3.7/site-packages/flask/app.py", line 2077, in wsgi_app
    response = self.full_dispatch_request()
  File "/root/miniconda3/envs/simple/lib/python3.7/site-packages/flask/app.py", line 1525, in full_dispatch_request
    rv = self.handle_user_exception(e)
  File "/root/miniconda3/envs/simple/lib/python3.7/site-packages/flask/app.py", line 1523, in full_dispatch_request
    rv = self.dispatch_request()
  File "/root/miniconda3/envs/simple/lib/python3.7/site-packages/flask/app.py", line 1509, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**req.view_args)
  File "flask_main.py", line 62, in sim
    result = json.dumps(res1,default=json_encoder)#,cls=NpEncoder
  File "/root/miniconda3/envs/simple/lib/python3.7/json/__init__.py", line 238, in dumps
    **kw).encode(obj)
  File "/root/miniconda3/envs/simple/lib/python3.7/json/encoder.py", line 199, in encode
    chunks = self.iterencode(o, _one_shot=True)
  File "/root/miniconda3/envs/simple/lib/python3.7/json/encoder.py", line 257, in iterencode
    return _iterencode(o, 0)
ValueError: Circular reference detected

The code is as follows:

@app.route('/sim',methods=['POST'])
def sim():
    try:
        res = MOC.make_order(**request.form)
    # print(res)
        res1 = {"response":200,'result':res}
    except Exception as e:
        res1 = {"response":404, 'result': [e]}
    print(61,res1)
    result = json.dumps(res1,default=json_encoder)#,cls=NpEncoder
    return result

Every time I restart the program, it works well again. I really don't know where the problem is. Can you help me? Thank you!