As i have noticed, with core theme i got this error while trying to login from my second pc "Your session timed out while posting. Please go back and try again"
To solve it i have changed to SMF Default curve theme and then tried again to login from second pc, and work successfully.
So i think some code is missing or not upgraded properly at core theme.
It is a bug.
in core theme's index.template.php find
<input type="hidden" name="hash_passwrd" value="" />
Change to
<input type="hidden" name="hash_passwrd" value="" /><input type="hidden" name="', $context['session_var'], '" value="', $context['session_id'], '" />
You can click on the "log in" button and it will be ok, I'm facing this problem too
perfect ,fixed!
you should at this code to next update.
install https://custom.simplemachines.org/mods/index.php?mod=4167
also next time please open your own topic for your issue.