I'm not seeing any issues with your site in my tests. Both single payments and recurring payments seam to be sending to PayPal without any error.

Please contact PayPal as the error you're getting makes no sense to their documentation. The error code DEPRECATED_PARAM_MODIFY doesn't even exist in their documentation. This seams to be an intermittent issue on their end. Seams like the deprecation of modify=0 was entirely a mistake though. The below is the only other link I've found with same issue and seams to have came to the same conclusion.

forums.wpsharks.com/t/buttons-stopped-working-deprecated-param-modify/7642/15

If this happens again remove the following line of code. Until we know what the heck PayPal is trying to do here we can't officially implement any kind of fix. Our implementation is in line with their documentation and until it says to stop sending modify=0 we'll have to keep sending it.

IN: /components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/processors/paypal/cbpaidsubscriptions.paypal.php
ON: Line 266
FROM:TO:
I do not recommend making this change unless absolutely necessary as we don't know what it could break by not sending it as it's intended to be sent with recurring payments to enable creating new subscriptions.