When migrating from QB 6.0.13 to 6.0.18 this error occurred:
2015-06-08 18:41:46,073 ERROR - Error migrating.
com.pmease.quickbuild.validation.ValidationException: not a well-formed email address (class:com.pmease.quickbuild.model.User, property:email, value:gavin dot wood at us dot panasonic dot com)
        at com.pmease.quickbuild.web.component.editor.EditContext.validate(EditContext.java:191)
        at com.pmease.quickbuild.web.component.editor.EditContext.validate(EditContext.java:140)
        at com.pmease.quickbuild.util.BeanUtils.validate(BeanUtils.java:367)
        at com.pmease.quickbuild.entitymanager.impl.DefaultDataManager.importData(DefaultDataManager.java:970)
        at com.pmease.quickbuild.Quickbuild.importData(Quickbuild.java:909)
        at com.pmease.quickbuild.bootstrap.Migrate$7.execute(Migrate.java:374)
        at com.pmease.quickbuild.Quickbuild$3.run(Quickbuild.java:627)
        at java.lang.Thread.run(Thread.java:745)
*********************** IMPORTANT ***********************
Migration process is failed and the data of previous version may got deleted from database. To restore the previous data, please delete and re-create the database and run below command:
/usr/lib64/quickbuild-6.0.13/bin/restore.sh /usr/lib64/quickbuild-6.0.18/migrate/2015.06.08_18.40.57.zip

The suggested restore did not work for the same reason.
I edited the Users.xml in the migrate zip file and changed two email entries from the format "alias at some dot domain dot com" to "alias@some.domain.com". and then followed procedures to remove/recreate the database, restore to 6.0.13 and retry the migration to 6.0.18, which subsequently worked.

Whenever a user logs into QB using the JIRA Authenticator, the email address will be malformed in this manner. You can correct the email address, but it will be malformed again at next logon.