HTTP 400 error when clicking search icon on all OAF pages on PROD

On PROD on any OAF / Web Page where we need to click on the search icon (e.g. AP Bank Accounts, AR Customers, AP Payment Profiles, Patch History in Oracle Application Manager), the pop-up page opens which normally allows us to run a search.

However, as soon as the pop-up opens, the window contains this in Internet Explorer:

And this in Firefox (same 400 Bad Request in Chrome):

Looking at an example of this issue - if we go to Payables Manager > Payments : Entry > Payments Manager

Go to Payment Process Requests tab.

Go to any form element which is linked to a magnifying glass - e.g. "Payment Process Request"