The TNS-12535 or ORA-12535 error is normally a timeout error associated with Firewalls or slow Networks. It can also be an incorrect listener.ora parameter setting for the CONNECT_TIMEOUT_ value specified. In essence, the ORA-12535/TNS-12535 is a timing issue between the client and server.

Error: ORA-12535 / TNS-12535 Text:
TNS: operation timed out
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Cause: The requested connection could not be completed within the timeout period specified by the CONNECT_TIMEOUT parameter in listener.ora. This error arises from the tnslsnr.

Action: Either reconfigure CONNECT_TIMEOUT to be 0, which means wait indefinitely, or reconfigure CONNECT_TIMEOUT to be some higher value. Or, if the timeout is unacceptably long, turn on tracing for further information.