I have gone through similar error. The error was not coming earlier, but recently I reinstall my oracle db and change the instance name from 'xe' to 'orcl', but forget to change this piece of code in property file:
spring.datasource.driver-class-name=oracle.jdbc.driver.OracleDriver
spring.datasource.url=jdbc:oracle:thin:@localhost:1521:***xe***
spring.datasource.username=system
spring.datasource.password=manager
Once I change it from 'xe' to 'orcl' everything is fine.