Two days back I ran into an issue where I was not able to login to my OEM12c test environment using any of the account. This happened after OEM database upgrade. Recently I did upgrade of my OEM database from 126.96.36.199 to 188.8.131.52 followed by upgrade to 184.108.40.206.
After upgrade was done successfully I tried to login to OEM using my account “dksharma” it gave me error, then I tried using the “sysman” but login failed again. All other users also reported the same issue.
I have document all my findings and solution to that in the attached document. If you also face the same Issue you use to document to fix your problem.