The assumption is that we are talking about windows 11, where the webui-user.bat file is configured to issue a random ngrok URL each time it is run.
The error log displayed when executing the webui-user.bat file is shown below.
Invalid ngrok authtoken? ngrok connection aborted due to: failed to connect session, Auth(Response("Your account is limited to 1 simultaneous ngrok agent session.\nYou can run multiple tunnels on a single agent session using a configuration file.\nTo learn more, see https://ngrok.com/docs/secure-tunnels/ngrok-agent/reference/config/\n\nActive ngrok agent sessions in region 'jp':\n - ts_2WDqiSYfqgrUbWYZsovMtaf68ZK (2400:4050:b6e0:1600:397e:59ae:c2dd:ef75)\r\n\r\nERR_NGROK_108\r\n"))
(1) I paid for ngrok, set ngrok's subdomain in ngrok's web browser, and obtained a static IP. ngrok was installed in the default directory.
(2) The subdomain of the static IP and the port of Stable Diffusion Web UI were described by editing the webui-user.bat file.
(3) After that, I ran ngrok start in one PowerShell and webui-user.bat in the other PowerShell.
(4) The one of ngrok start was executed without any problem.
(5) The webui-user.bat got the error I mentioned earlier.
The authtoken for ngrok was not wrong.
I would like to resolve the above error. If anyone knows how to solve this problem, I would like to know so that even a beginner can understand.