Hi,
The Paypal recurring plugin is acting the same way than the HikaShop classical paypal plugin, but it will use some extra parameter and a different "cmd" for the subscription.
Regarding the URL, it's always :
sandbox.paypal.com/cgi-bin/webscr
So I suppose that the browser is redirected to other pages and one of them generates the problem.
Now, when the plugin is configured with the "debug" mode, you do have debug data in the HikaShop payment log file.
There you will have some raw data of the content which is submitted to the Paypal platform, it allows to understand what is send and see if there is an issue. It always useful when you talk with the Paypal support team so you can tell them what is the data.
Regards,
Jerome - Obsidev.com
HikaMarket & HikaSerial developer / HikaShop core dev team.
Also helping the HikaShop support team when having some time or couldn't sleep.
By the way, do not send me private message, use the "contact us" form instead.