Oracle SqlError: Protocol Violation / OALL8 is in an inconsistant state
SQL Developer uses the ORACLE_HOME and it's drivers when available. The issue you are seeing in this is that the ORACLE_HOME is not the same version as the JDBC Driver bundled in SQL Developer.
The 'fix' for this is to ensure the oracle client is up to date w/ the version of sqldev you have installed. The easiest way to find this is when connection run show jdbc
show jdbc-- Database Info --Database Product Name: OracleDatabase Product Version: Oracle Database 12c Standard Edition Release 12.1.0.2.0 - 64bit ProductionDatabase Major Version: 12Database Minor Version: 1-- Driver Info --Driver Name: Oracle JDBC driverDriver Version: 12.2.0.1.0Driver Major Version: 12Driver Minor Version: 2