IllegalStateException: Unable to create project: Errors: {} Error Messages: [Invalid default Assignee.]

Symptoms:

Caused by: java.lang.IllegalStateException: Unable to create project: Errors: {}
Error Messages: [Invalid default Assignee.]
    at com.botronsoft.jiracompatibility.impl.JiraCompatibility7.createProject(JiraCompatibility7.java:143)
    at com.botronsoft.jiracompatibility.impl.JiraCompatibilityFacade.createProject(JiraCompatibilityFacade.java:98)
    at com.botronsoft.jira.rollout.impl.merge.ConfigurationRootMergeAdapter.addProject(ConfigurationRootMergeAdapter.java:353)
    at com.botronsoft.jira.rollout.impl.merge.di.CacheRefreshInterceptor.invoke(CacheRefreshInterceptor.java:30)
    at com.botronsoft.jira.rollout.impl.merge.ConfigurationRootMergeAdapter.addFeature(ConfigurationRootMergeAdapter.java:116)
    at com.botronsoft.jira.rollout.impl.merge.AbstractMergeAdapter.notifyChanged(AbstractMergeAdapter.java:65)
    ... 20 more

Cause:

The setting for allowing unassigned issues differs on the source and target systems.

JIRA 6 ref: https://confluence.atlassian.com/jira061/jira-administrator-s-guide/configuring-global-settings/configuring-jira-options

Resolution:

Unify the setting on both source and target. Go to the Jira Administration cogwheel -> System -> General Configuration -> Allow unassigned issues.