It seems like i was unable to replicate this issue, you can watch my video : http://take.ms/WGPJ1

Based on what i research, most of the possibility issue caused by this Joomla Plug-in: System - Page Cache , you can refer on this referring link.

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

This is happening, if you have a the login page open on your browser, and you keep it open, and try a login after it was open for a while (maybe 10 min or more).

Our page cache plugin is disable.

I've tested several times login on your site, it seems work fine for me.

This is how i test just now :
1. open your homepage
2. put it there around 10 ~ 30 minute
3. at the meantime, i assist other customer fix their current issue
4. After that I come back on your site without refresh the page, enter username and password then click login
5. login successfully

I get this same problem all the time.

All site cache is disabled.

Hey John,

Can you start a new thread on our forum and provide us with your site backend and FTP access at https://stackideas.com/dashboard/site so we can better have a check.