Payment methods link not working in admin after upgrade
I have just upgraded a 1.7 CE site to 18.104.22.168. Everything is fine except when I try to access "Payment Methods" under Sales in the config area. When clicking the link (/system_config/edit/section/payment/key/76ea0f2b9935f6828fc71675351e0dd9/) it hangs for a while and redirects me back to the front-end homepage. There are no error messages. This only happens in the "Default Config" view, underneath current config scope.
Then when trying to use our payment method on the front end (Paypal standard), nothing happens when trying to complete the order.