Correct me if I am wrong but when opening Server Manager, if you don't have a SID set or the SID you have set does not exist on the server you get the ORA-12203 error
When you are connecting you are using the @ in your connect string so it finds the SID you want to connect to from your TNSNAMES file
To stop this error is can do this before opening server manager
set ORACLE_SID=
then
svrmgrl
connect internal without the @
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