Are you using LDAP??
Have faced this issue before and found that the issue resides within LDAP.
We just reconfigured the connection of LDAP and waited for sometime for the configuration to commit and everything was back to normal. We too thought that the issue resides with the ORACLE database and did also the steps you mentioned but the issue reoccur from time to time.
Also in another instance, we faced this issue with MSSQL databse. Also we found the in the MSSQL DB properties that the users were locked, there is an option ther to unock them but also the isue reoccur from time to time. Uninstall/reinstall the patch solved our issue.