ORA-609 TNS-12535: TNS:operation timed out, Client address: <unknown>

Hello,

Oracle 11.2, SE, no patches.

I got today two such errors in alert log file. The problem is that usually I got details about Client address like:

Client address: (ADDRESS=(PROTOCOL=tcp)(HOST=10.10.10.10)(PORT=34567)) so I could track this with listener log to get more info about affected processes.

This time I got unknown address. Any idea what it can mean ?

Currently we have occurrences for such error in alert log file. APR: 87, MAY 9

Time: 04-AUG-2020 14:05:43

   Tracing not turned on.

   Tns error struct:

     ns main err code: 12535

TNS-12535: TNS:operation timed out

     ns secondary err code: 12606

     nt main err code: 0