Re: ORA-12537: TNS:connection closed (DBD ERROR: OCIServerAttach).
regarding this error, I heard similar error messages for one of databases on the Grid control. However nothing from the thread resolved my issue. I believe this issue originates when the management server cannot contact the managed node or vise-versa for whatsoever reason. Most often time the error is captured in the emagent.trc file located at the AGENT_HOME/sysman/log. My own case was an expired password of the DBSNMP user which is used by the agent to connect to the managed database. Immediately I changed the expired password grid control connected seamlessly and the error cleared off.
Hope this helps someone