I had the same problem and solved it:
PROBLEM: Cannot disable, edit, remove any port mirroring sessions. Received error, similar to:
"The object or item referred could not be found. vDS dvs port 463 cannot be found".
CAUSE: Although I had three port mirroring sessions configured, only one of the port mirroring sessions contained a source or destination "port 463" in it's configuration. This caused problem with any/all other port mirroring sessions.
SOLUTION: Find the one port-mirroring session that contains the port number from the error ("port 463 cannot be found") and edit the source/destination by deleting the offending port number. Once the offending port number has been removed from the one specific port mirroring session, you will be able to edit/delete all the other port mirroring sessions.