Some of the methods for EJB service "UserTaskServiceEJBLocal" like "saveContentFromUser" (to update output dynamic fields), always throw an error "User '[UserImpl:'xxxxx']' does not have permissions to execute operation 'Modify' on task id xxxxx" if custom usergroupcallback is used.

From the stack trace shared it seems like "CommandBasedTaskService.addContentFromUser" is triggered [[1]|https://github.com/kiegroup/jbpm/blob/8e3ea62e6a98e765c7bd58b9d88b591b167d3c46/jbpm-human-task/jbpm-human-task-core/src/main/java/org/jbpm/services/task/impl/command/CommandBasedTaskService.java#L590
].

at org.jbpm.services.task.commands.TaskCommandExecutorImpl.execute(TaskCommandExecutorImpl.java:49)
at org.jbpm.services.task.impl.command.CommandBasedTaskService.addContentFromUser(CommandBasedTaskService.java:591)
at org.jbpm.kie.services.impl.UserTaskServiceImpl.saveContent(UserTaskServiceImpl.java:734)
at org.jbpm.kie.services.impl.UserTaskServiceImpl.saveContentFromUser(UserTaskServiceImpl.java:710)

It looks like some of the commands from [2] including "addContentFromUser" [3] are extending "TaskCommand<Long>", while others are extending "UserGroupCallbackTaskCommand<Void>" [4].

Complete stack trace is available here.