Take a look at the security section in concrete/config/concrete.php file and on line number 987 try setting the 'invalidate_on_ip_mismatch' to false

Thanks for your reply and suggestion but it did not make any difference.

could you check if, by any chance, you have to token fields on the login page? They are hidden input fields with name ccm_token. You can look for them in the page's source

i found the hidden token field in the page code on line 104:
<input type="hidden" name="ccm_token" value="1570807514:5e8c698f456e436dbf1d33d6619dcb7c" />

However, what is weird is I've just signed into c5 community site (this site) and the first attempt gave me a token error. So the problem could be my browser maybe??

Im using chrome osx, so have just tried using firefox osx to login and i didn't get the error.