Error : This page isn’t working if the problem continues, contact the site owner. HTTP ERROR 400

Hello everyone, after making some changes in the nameservers and I tried to stop the connection with Cloudflare, but the host told me that the connection with Cloudflare is mandatory, so when I went back to the connection again, I had some problems, including that when I press the login button on the login page, it shows a 400 error and some devices The site does not work for them until after turning on the VPN or changing the DNS. What is the solution to my problem, knowing that my site is programmed in Asp.net language