Issue:
As shown in the pic below, when I was trying to logging into my account on Nexcloud via web interface, the page stops responding a few seconds after loading, the password field clears, and I can’t login. The username and password are promised to be correct.
Please read the Brute Force Protection chapter and review your Nextcloud log a bit. You likely either have another client connecting from the same source IP address or you have a proxy misconfiguration that is making all connections appear from the same IP address. There are also some other possibilities. It might be helpful to temporarily adjust your loglevel to 1 (INFO level) as well to see all brute force attempts detected earlier in the cycle.