Restore backup fails with "Import failed. null" error

This causes this exception in the logs:

java.lang.NullPointerException
at com.atlassian.confluence.importexport.impl.FileBackupImporter.importConfigDirectory(FileBackupImporter.java:416)
at com.atlassian.confluence.importexport.impl.FileBackupImporter.importEverything(FileBackupImporter.java:108)
at com.atlassian.confluence.importexport.impl.BackupImporter.doImport(BackupImporter.java:92)
at com.atlassian.confluence.importexport.impl.FileBackupImporter.doImport(FileBackupImporter.java:41)
at com.atlassian.confluence.importexport.DefaultImportExportManager.importAs(DefaultImportExportManager.java:74)
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:324)
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 $Proxy48.importAs(Unknown Source)