Hi,
We got all your accesses and had an extensive look at the problem today.
First, on your copy of your website, I didn't get the same error message as you. I only got the error message: "Save failed with the following error: "
The error is apparently linked to the line:
if(@$_REQUEST['ctrl']!='checkout') return false;
in the file plugins/system/hikashopuser/hikashopuser.php
Actually, that line has been removed in recent versions of HikaShop as it was not necessary and created some issues in some cases.
Removing that line fixed the error message on your copy website.
Updating to the latest version of HikaShop would have also fixed the problem.