It's the return_url erroring. It's built using the following.
Code:
$_CB_framework->pluginClassUrl( [ 'plugin' => 'cbpaidsubscriptions', 'do' => 'displayplans' ], false )
So that would be the menu item with a menu type of Community Builder > Plugin with Plugin set to CB Paid Subscriptions and Do set to Display Plans.
Strange their API is complaining about this though. It's sent in the POST body so it shouldn't need urlencode applied to it as it's not sent as GET. Please make the following change and confirm if this fixes your issue. If it does will commit the fix for next build release.
IN: components/com_comprofiler/plugin/user/plug_cbpaidsubscriptions/processors/stripe/cbpaidsubscriptions.stripe.php
ON: Line 3183
FROM:
Code:
'return_url' => $_CB_framework->pluginClassUrl( [ 'plugin' => 'cbpaidsubscriptions', 'do' => 'displayplans' ], false ),
TO:
Code:
'return_url' => urlencode( $_CB_framework->pluginClassUrl( [ 'plugin' => 'cbpaidsubscriptions', 'do' => 'displayplans' ], false ) ),