During a space export you can sometimes get this stack trace:

com.atlassian.confluence.importexport.ImportExportException: Couldn't backup database data.
at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEntities(AbstractXmlExporter.java:181)
at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEverything(AbstractXmlExporter.java:85)
at com.atlassian.confluence.importexport.impl.FileXmlExporter.backupEverything(FileXmlExporter.java:73)
...
Caused by: java.lang.NullPointerException
at com.atlassian.confluence.security.ContentPermission.compareTo(ContentPermission.java:210)
at java.util.TreeMap.compare(TreeMap.java:1093)
at java.util.TreeMap.put(TreeMap.java:465)
at java.util.TreeSet.add(TreeSet.java:210)

NB. Mostly you just get this error:
'The page you were trying to reach does not exist. You may want to try a search, or browse the site to find the page you were looking for.'