The error page for the Tomcat ErrorReportValve is not configurable.
There are errors like invalid URls which trigger the internal error page to be presented.
This way everybody knows a Tomcat is serving the website (information disclosure).
Example URL: http://localhost:8080/[test
The tomcat ErrorReportValve can be configured by creating a custom valve (see TomcatErrorReportValveConfiguration class).
It would be easier to have the error page configured by the application.properties.
Activate the customerrorvalve profile to run with a custom error valve.
See issue: spring-projects/spring-boot#21257