Hi,
It's not a HikaShop issue.
Now it might be an issue of how you configured the PayPal payment method.
I can see you've activated the "sandbox" setting of the payment method.
But the email address you entered in the payment method looks like your real PayPal account email address and not a sandbox account email address. That might lead to this error. But it could be something else, like some setting of the sandbox account not being configured properly.
The best would be to check with the PayPal support what is wrong. The PayPal plugin itself is still working fine, that's for sure.