Exporting a space that contains a page level permission can throw an exception like this one:

CONFLUENCE
System Error
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1. a description of your problem and what you were doing at the time it occurred
2. cut & paste the error and system information found below
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Cause:
com.atlassian.confluence.importexport.ImportExportException: Couldn't backup database data.
at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEntities(AbstractXmlExporter.java:181)
caused by: java.lang.NullPointerException
at com.atlassian.confluence.security.ContentPermission.hashCode(ContentPermission.java:257)

Stack Trace: [hide]

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)
at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.doExport(AbstractXmlExporter.java:78)
at com.atlassian.confluence.importexport.impl.FileXmlExporter.doExport(FileXmlExporter.java:40)
at com.atlassian.confluence.importexport.impl.XmlExporter.doExport(XmlExporter.java:24)
at com.atlassian.confluence.importexport.DefaultImportExportManager.exportAs(DefaultImportExportManager.java:111)
at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:39)
at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:25)
at java.lang.reflect.Method.invoke(Method.java:585)
at org.springframework.aop.support.AopUtils.invokeJoinpointUsingReflection(AopUtils.java:284)
at org.springframework.aop.framework.ReflectiveMethodInvocation.invokeJoinpoint(ReflectiveMethodInvocation.java:155)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:122)
at org.springframework.transaction.interceptor.TransactionInterceptor.invoke(TransactionInterceptor.java:56)
at org.springframework.aop.framework.ReflectiveMethodInvocation.proceed(ReflectiveMethodInvocation.java:144)
at org.springframework.aop.framework.JdkDynamicAopProxy.invoke(JdkDynamicAopProxy.java:174)
at $Proxy49.exportAs(Unknown Source)
at com.atlassian.confluence.importexport.actions.ExportSpaceAction.execute(ExportSpaceAction.java:111)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:168)
at com.opensymphony.xwork.interceptor.AroundInterceptor.intercept(AroundInterceptor.java:35)
at com.opensymphony.xwork.DefaultActionInvocation.invoke(DefaultActionInvocation.java:165)
at com.opensymphony.xwork.interceptor.DefaultWorkflowInterceptor.intercept(DefaultWorkflowInterceptor.java:55)
at com.atlassian.confluence.core.ConfluenceWorkflowInterceptor.intercept(ConfluenceWorkflowInterceptor.java:39)