It happened to me the other day: I could no longer login to an Enterprise Manager site, and I was sure I had the correct password. It might have something to do with running ssoReplSetup.jar.
Anyway, the solution is to use a hidden option of emctl: reset. These are the steps to revitalize your OEM:
- edit $ORACLE_HOME/sysman/j2ee/jazn-data.xml
- find ias_admin entry
- remove line with "credentials"
- save file
- emctl set password reset <new_password>
- (re)start oem: emctl start iasconsole
You can now login again, using <new_password>.