Trouble calling OA Framework forms from within Oracle Apps

I have an R12.0.4 instance of E-Business Suite - it's a Vision demo instance - which has been working just fine for many months.  I haven't changed any configuration or profile options recently but now for some reason I am unable to open any of the OA Framework forms (e.g. Open Purchasing Periods, Accounting Setup Manager) from within the application (this has previously worked just fine). I can access these forms if I navigate straight there from the personal home page but if I go into Oracle Forms first and then navigate to the OA Framework form, it re-calls the login