I think you would need FLASHBACK ANY TABLE privilege to see a flashback view of data in another schema - in this case SYS, which owns obj$, source$ etc. Alternatively connect as the SYS user, although most sites understandably won't let you do that unless you're a DBA. Then you could try calling
DBMS_FLASHBACK.ENABLE_AT_TIME(). (Untested as I'm not near a system where I have privileges right now.)