Hi suibee
Our template should work fine with PHP 7.2, are you using any 3rd party extension for checkout on your site?
suibee
Could you share the admin + ftp account? I will check for you
Is there any cache settings on your server side? I tried switch default template to Protostar to check how the checkout proceeds but it keeps showing with GK Quark?
Could you check the same?
Could you share the admin + ftp account again?