PayPal login problem ..version Rollback ??

I recently discovered that my paypal gateway is not working. I am getting errors from Paypal saying the login cant be processed.
I have the same system running on another domain running 4.1.6 and it works fine. I think somewhere in between the upgrades WP has broken the logic in the theme.
1) How big a deal is it to rollback to 4.1.6 temporarily while this problem is vetted?

2) has anyone else had this issue?
Thx..