We recently bought your product Membership Pro, and we are starting to configure it now. We also purchased the Redsys plugin, which is the system that works in Spain.
Here is the problem: when we try to subscribe, an error appears (see screenshot 1) in the transaction. In the source code we can see that this is the SIS0075 error (screenshot 2), which is due to an incorrect string for the Ds_Merchant_Order field (it should be 12 digits maximum and the first 4 should be numbers, according to Redsys instructions, see screenshot 3).
I just made some modification to the payment plugin to meet that requirement. Could you please access to My Downloads menu item, download latest version of the payment plugin, install it and try again. It should work as expected after the update