At least two users have reported this same stack trace when trying to backup their data:

org.springframework.dao.DataIntegrityViolationException: (SessionSynchronization): data integrity violated by SQL 'null'; nested exception is:
java.sql.SQLException: Integrity constraint violation: FK932472461E2E76DB table: OS_GROUP in statement [insert into os_user_group (user_id, group_id) values (6, 4)]
java.sql.SQLException: Integrity constraint violation: FK932472461E2E76DB table: OS_GROUP in statement [insert into os_user_group (user_id, group_id) values (6, 4)]
at org.hsqldb.Trace.getError(Unknown Source)
at org.hsqldb.jdbcResultSet.(Unknown Source)
at org.hsqldb.jdbcConnection.executeStandalone(Unknown Source)
at org.hsqldb.jdbcConnection.execute(Unknown Source)
at org.hsqldb.jdbcStatement.fetchResult(Unknown Source)
at org.hsqldb.jdbcStatement.executeUpdate(Unknown Source)
at org.hsqldb.jdbcPreparedStatement.executeUpdate(Unknown Source)
at net.sf.hibernate.impl.NonBatchingBatcher.addToBatch(NonBatchingBatcher.java:22)
at net.sf.hibernate.collection.AbstractCollectionPersister.recreate(AbstractCollectionPersister.java:1075)
at net.sf.hibernate.impl.ScheduledCollectionRecreate.execute(ScheduledCollectionRecreate.java:23)
at net.sf.hibernate.impl.SessionImpl.executeAll(SessionImpl.java:2326)
at net.sf.hibernate.impl.SessionImpl.execute(SessionImpl.java:2283)
at net.sf.hibernate.impl.SessionImpl.flush(SessionImpl.java:2180)
at org.springframework.orm.hibernate.SessionFactoryUtils$SessionSynchronization.beforeCommit(SessionFactoryUtils.java:217)
at org.springframework.transaction.support.TransactionSynchronizationManager.triggerBeforeCommit(TransactionSynchronizationManager.java:172)
at org.springframework.transaction.support.AbstractPlatformTransactionManager.commit(AbstractPlatformTransactionManager.java:213)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:172)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:196)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:135)
at $Proxy21.exportAs(Unknown Source)
at com.atlassian.confluence.importexport.actions.BackupAction.execute(BackupAction.java:29)