I can not explain in detail why this happens but it is Joomla default behaviour when the site is open for a long time without logging in. You have to refresh the page then before being able to login. Therefor this is Joomla related and not extension related.

Hey there,

I am really sorry for the delay of this reply as it is a weekend for us here.

If I am not wrong the reason why you hitting this most likely is the cache system cached the login security token on the page, so when you trying to login, that cached token already used from one of your existed logged user, your login will failed, so once you refresh the page, it will regenerate the token again, so you can able to login.

From other thread i have disabled that "System - page cache" plugin, it should work fine.

Reference link : https://forum.joomla.org/viewtopic.php?t=930748

By the way, thanks for help Sabih