Lots of login errors with several logins

This is a problem that can only be confirmed on one PC(Windows10(1703)). There are four other similar PCs, but there are no problems with them. *However, the login problem in Firefox which I posted before is still there. I will ignore this time.

Login from any web browser(Edge, Chrome, Vivaldi, Firefox, Waterfox, (Windows explorer) network drive. I will not ask these questions. First time login fails. There is no mistake in the input information surely. Then, login succeeds for the second time or the third time. At that time, when I look at the log, “login failure” is displayed 13 to 15 times consecutively. I have logged in only a few times. I have never logged in more than 10 times.
I have set up fail2ban, and this error causes everyone to be refused connection.

Only my nextcloud is running on my server. Such as Dropbox and other online storage services can be used without problem.

I doubted the setting of Windows and tried several settings, but had no effect at all. Resetting the network settings, deleting the cache related to the network setting, moving to a different network, etc. Do you know what is causing this? Please give me a hint.

Nextcloud: 13.0.1 / Nginx 1.11.10 / PHP 7.1.17 / CentOS 7.3 / MariaDB 5.5.56