This error is also visible on magnolia website. Please look at
http://www.magnolia-cms.com/home/our-company/news/press-releases/old-press-releases/magnolia-4-0-released.html

Maybe the affected nested document level is a noticable information for you. The error occurs at 6th document level, so does mine.
Is there any need to access the mentioned "extra" object?

My personal error stack trace is the following:

FreeMarker template error!

get(extras) failed on instance of info.magnolia.module.templatingkit.templates.STKTemplateModel
The problematic instruction:
----------
==> list model.extras.column[columnIndex] as extra [on line 17, column 5 in templating-kit/templates/global/extrasArea.ftl]
in user-directive extrasColumn [on line 13, column 5 in templating-kit/templates/global/extrasArea.ftl]
in include def.extrasArea.template [on line 52, column 25 in templating-kit/templates/main.ftl]
----------

Java backtrace for programmers:
----------
freemarker.template.TemplateModelException: get(extras) failed on instance of info.magnolia.module.templatingkit.templates.STKTemplateModel
at freemarker.ext.beans.BeanModel.get(BeanModel.java:223)
at freemarker.core.Dot._getAsTemplateModel(Dot.java:76)
[...]
Caused by: info.magnolia.cms.security.AccessDeniedException: User not allowed to (Add, Set, Read) path [/level1/level2/level3/level4/level5/level6/extras]
at info.magnolia.cms.core.Access.isGranted(Access.java:64)
at info.magnolia.cms.core.DefaultContent.<init>(DefaultContent.java:156)
at info.magnolia.cms.core.DefaultContent.createContent(DefaultContent.java:194)
at info.magnolia.cms.util.ContentWrapper.createContent(ContentWrapper.java:188)
at info.magnolia.cms.core.AbstractContent.createContent(AbstractContent.java:75)
at info.magnolia.cms.util.ContentUtil.getOrCreateContent(ContentUtil.java:195)
at info.magnolia.module.templatingkit.templates.ExtrasAreaModel$1.exec(ExtrasAreaModel.java:71)
at info.magnolia.module.templatingkit.templates.ExtrasAreaModel$1.exec(ExtrasAreaModel.java:70)
at info.magnolia.context.MgnlContext.doInSystemContext(MgnlContext.java:396)
at info.magnolia.context.MgnlContext.doInSystemContext(MgnlContext.java:371)
at info.magnolia.module.templatingkit.templates.ExtrasAreaModel.<init>(ExtrasAreaModel.java:69)
at info.magnolia.module.templatingkit.templates.STKTemplateModel.getExtras(STKTemplateModel.java:148)
... 129 more