I have been getting help from Paypal technical support on tracking down a strange bug where some users are not redirected after payment to our site for some reason, but most users are.
The really helpful support guy at Paypal said the following:
We have checked those 5 PayKey respectively, it showed that all the customers/buyers were US users. And in AP-6PC49991TU1544250, the return URL was [REDACTED], we visit it successfully on PC(with a VPN), but failed on a iPhone(We are here in China). To avoid/fix this issue, please make sure that this URL can be visited by US users.
I am wondering how it is possible that in some locations or devices a URL can not be redirected to?
One thing to note is that the page being redirected to also performs a redirect using Codeigniter redirect method.
So Paypal redirects to a page, it receives a 302 with a location and then I guess it's up to the browser to redirect. Maybe some browsers do not support this?
You may have an error with your Intermediate Certificate not being compliant with certain mobile operating systems. Personally, I check my client's SSL with SSL Checker. The HTTP response status code 302 Found is a common way of performing URL redirection, but if the SSL is non-compliant with the chain provided by the issuer and attempting to perform a redirect; it will fail. Contact your issuer if you receive this error to supplement an up-to-date Intermediate Certificate.