How to address ORA-03113?
This is a generic error.

If you receive this in an active session while running some query, PL/SQL code or other statements, you should check the alert log of the database for further information. Usually it is caused by critical errors such as ORA-07445 that cause the server process that belongs to your session to terminate unexpectedly.

But if you receive this error in and idle session after a specific amount of time, as in your case, then do not worry, because it is absolutely normal when the database enforces an inactivity timeout, for example in the case of DRCP (Database Resident Connection Pooling).

Here is an example:

Database Resident Connection Pool (DRCP)