The error in the logs may look like this:

com.atlassian.confluence.importexport.ImportExportException: Couldn't backup database data. at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEntities(AbstractXmlExporter.java:254) at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.backupEverything(AbstractXmlExporter.java:93) at com.atlassian.confluence.importexport.impl.AbstractXmlExporter.doExport(AbstractXmlExporter.java:66)

......

ERROR [net.sf.hibernate.proxy.LazyInitializer] Exception initializing proxy
net.sf.hibernate.ObjectNotFoundException: No row with the given identifier exists: 1853, of class: com.atlassian.confluence.core.ContentEntityObject
at net.sf.hibernate.ObjectNotFoundException.throwIfNull(ObjectNotFoundException.java:24)
at net.sf.hibernate.impl.SessionImpl.immediateLoad(SessionImpl.java:1936)
at net.sf.hibernate.proxy.LazyInitializer.initialize(LazyInitializer.java:53)
at net.sf.hibernate.proxy.LazyInitializer.initializeWrapExceptions(LazyInitializer.java:60)
at net.sf.hibernate.proxy.LazyInitializer.getImplementation(LazyInitializer.java:164)
at net.sf.hibernate.proxy.CGLIBLazyInitializer.intercept(CGLIBLazyInitializer.java:108)
at com.atlassian.confluence.core.ContentEntityObject$$EnhancerByCGLIB$$87fec53b.getOriginalVersion(<generated>)
at sun.reflect.GeneratedMethodAccessor133.invoke(Unknown Source)