We’ve hit strange error ORA-01012 “not logged on” after trying to re-enable DGMGRL after upgrading to 12c. Diagnosis took some time and the error was completely spurious as it come out. One of the AFTER LOGON TRIGGERs was not ignoring SYSDG user/role (new DataGuard role for users separation in 12c). The solution is simply to add SYSDG in excluded users list (in addition to SYS and PUBLIC roles for previous versions). Hope this helps others that are in hurry with upgrades.