My answer to your stackoverflow post explains why it might be different for you in 6.2 and how you can disable the security manager.
This exception comes from the Java Security manager. If you are using the Alfresco docker images, the security manager is enabled by default. If you have done a non-docker installation, you are launching tomcat using the "-security" switch. While the security manager is recommended because it makes your installation more secure, it is not required.
If you are using Alfresco's docker images, you will need to modify the docker-compose.yml file and specify a "command" argument for the "alfresco" service. Out of the box, the "command" argument is ["catalina.sh" "run" "-security"]. You need to remove the "-security" flag. For example:
command: ["catalina.sh" "run"]
If you have installed Alfresco manually, you simply need to change the way that you are starting it. Remove the "-security" flag.
Using parallel execution causes threads to spawn with no security configuration. This means that for this situation, you cannot use parallel execution unless you disable the security manager. So your choices are to disable the security manager using the above steps or to use serial execution.