Spring Boot Web application working well in STS's embedded Tomcat 9 server but not on Linux's Tomcat 9 installation

I've got a Spring Boot Web application which is basically a registration form for signing users up and store the records in a MySQL database.

I'm using Spring Tool Suite 3.9.11 IDE, in which I have configured a Tomcat 9 server. This application works perfectly well if I run it from the IDE, but whenever I try to deploy it on a "real" Tomcat server, at the precise moment the application has to do the connection to the MySQL server, it crashes.

If you please can help me, I would really appreciate it.

Thanks very much in advance.