Backup import fails

I use backup and restore option for importing spaces from confluence to another (by changing the buildNumber in the export descriptor properties).
Currently I can not import spaces because the hosted version is 5.2 while the latest standalone is 5.1. The following error message is returned:

Import failed. Check your server logs for more information. com.atlassian.confluence.importexport.ImportExportException: Unable to complete import: Error while importing backup: not-null property references a null or transient value: com.atlassian.confluence.mail.notification.Notification.userName