ora-03114 not connected to oracle Since the upgrade to the 19C Oracle DB (before 12C) I receive randomly those ORA-03114 error message's.
Before the upgrade I never received it. Any idea's what exactly why those error messages pops up suddenly and what changes needs to be done?

Domain Win10 PC
Not all PC's in the domain having this issue
only localy tnsnames.ora change

ORA-03114: not connected to ORACLE
ORA-03114 not connected to ORACLE
Cause: A call to Oracle was attempted when no connection was established. Usually this happens because a user-written program has not logged on. It may also happen if communication trouble causes a disconnection.
Common causes for the ORA-03114 error include:

Listener process is not running
DB is shutting down or session has been forcibly disconnected
Missing entry to the tnsnames.ora
Network issues such as timeouts
Problems with connection pooling (shared servers, MTS)

If - as you seem to suggest - your tnsnames.ora was modified recently, an incorrect configuration could be one possible reason for the error, especially if other clients are not experiencing any problems. Compare the tnsnames.ora and sqlnet.ora files between the client that fails and the clients that work. Also check the network path for routers or firewalls that could be enforcing idle timeouts, if that is applicable in your environment.
See here for further explanation:

https://www.tekstream.com/resource-center/ora-03114-not-connected-to-oracle/
http://www.dba-oracle.com/t_ora_03114.htm