Order cannot be confirmed with the PayPal Recurring Payment Plugin

-- HikaShop version -- : 4.4.3
-- Joomla version -- : 3.9.27
-- PHP version -- : 7.4
-- Browser(s) name and version -- : Firefox 90.0.2

Hi, we currently have a bit of trouble with the PayPal Recurring Payment Plugin.
We use HikaSerial to generate serial numbers and we also use it for subscriptions. When a user wants to buy a subscription, he needs to use the recurring payment plugin for PayPal. Of course everything is in sandbox mode, since the shop isn't open yet.
The checkout process works up to the point where the payment needs to be confirmed. PayPal accepts the subscription and tranfers the money. But it cannot confirm the successful transaction to hikashop. The order can't be confirmed and the subscription stays closed. I enabled the debug mode for the plugin and get some "404 ROUTE_NOT_FOUND" errors at the end.
The weird thing is, that this only happens for the recurring payment plugin. We also offer other products without an attached subscription and use the basic PayPal Plugin for them, which works just fine. And the settings for both plugins are pretty much identical.

Sometimes I also get the following error:
Deprecated: Function get_magic_quotes_gpc() is deprecated in /plugins/hikashoppayment/paypal_recurring/paypal_recurring.php on line 770

Hi Jerome.

Yes, we are also using the Hikashop Paypal Plugin. However, we don't get any errors with it. The error only happens with the recurring payment plugin.

Yes, I contacted them. They couldn't give me an answer right away, so I'm not sure what caused the error. However I guess there was an error in the account settings, because I solved the problem by creating a new personal sandbox account.
Since today I couldn't login into the old sandbox account, so I created a new one. Also tested the subscription and now it works like intended.

Thanks for the help.