Magento does not eliminate a double slash after the domain name.
You can see the bug on any Magento website, e.g.:
https://marketplace.magento.com/tnw-quickbooks-full.html
https://marketplace.magento.com//tnw-quickbooks-full.html
The solution for Nginx:
server {
<…>
merge_slashes off;
rewrite (.*)//+(.*) $1/$2 permanent;
<…>
}