As SA_MULTIFISCALYEARS has been enabled (logged out and logged in again after clearing browser cache), verify the setting is saved in Setup => Access Setup => Role: System Administrator (Role of logged in user).
The function that allows Suppliers, Customers and Items to be entered in older fiscal years is defined in admin/db/fiscalyears_db.inc is is_date_in_fiscalyears() and is used in the function is_date_in_fiscalyear() defined in includes/date_functions.inc where the first said file is "include"d.
Inspect the variables at that point in Line 204 of the last file stated above. The function user_check_access() is defined in includes/current_user.inc file.
Scope, availability and visibility of the variables and their values will be of use in investigating it. If all is well, then scour the scripts that use these values before "post"ing the updated values to the db.