Plugin causes login page to fail intermittently

Having your plugin activated on my site causes the user login page to fail approximately 50 percent of the time.

Behaviour 50 percent of the time is that the user enters their username and password and the user is logged in, or if they entered the password wrong, they are told the password is wrong.

the other 50% of the time, the user enters credentials and clicks “Log In” and the page is just refreshed and nothing happens. The user is NOT logged in, or told their password is wrong, and has to enter their credentials again.

I made a video of this happening. Disabling the plugin seems to cure it immediately.

Please could you fix this?