Update ERPNext and ERR_CONNECTION_REFUSED

Hi all,
I’m new to ERPNext and I have installed to run on my server. After few days of learning, I tried to turn to developer mode to change a table, but then unchaged due to a warning from old user and turned off developer mode. However when I update ERPNext, the system did not allow me to update unless I have to do hard update by using RESET. I did it and found that I could not access to my site, the connection refused.
I tried to figure it out in many ways and read a lot of similar topics but still not resolve the problem. However, when I tried the command “bench serve --port 8000” and keep it running in debug mode then I can login very fast and can do the works (even have more than error logs about frappe.email.queue.flush). When I turn off “bench serve --port 8000” then the connection stopped working again until I run that command to connect.
I don’t know how to resolve this problem, so please give me an advice
Thank you

Hello @minhtran

Please provide further details like error logs and better with a screenshot.

Dear @iRaySpace
I have attached a typical error message. There are hundred of them in my server log
Thank you for reply

And this is how the command “bench serve --port 8000” looks like