If I try to start or stop container deployed on kie server using Server Management page in kie drools workbench and I don't have defined user "kieserver" I will get error shown in attachment.

It is caused by using hardcoded values for controller username and password.
See https://github.com/droolsjbpm/kie-wb-common/blob/6.3.x/kie-wb-common-screens/kie-wb-common-server-ui/kie-wb-common-server-ui-backend/src/main/java/org/kie/workbench/common/screens/server/management/backend/ServerManagementServiceImpl.java#L101#L102