Hey there,
I already tested this on different browser in Windows and Mac machine, it seems like that is random issue, some time chrome browser also hitting this warning pop up, actually that main error is "Invalid Token".
I suspect some where on your site did cached those HTML content on the site so that you hitting this issue is because token didn't renew to new token, it keep re-use back the same token.
Can you provide us with your site backend access so we can better have a check and see which cache system doing this?
Because when i access your site backend site.com/administrator, i hitting following error :
Forbidden
You don't have permission to access /administrator/ on this server.
And your FTP access as well.
Command: AUTH TLS
Response: 234 AUTH TLS successful
Status: Initializing TLS...
Error: Received TLS alert from the server: Handshake failed (40)
Error: Could not connect to server