On Magnolia 4.4.x area ftl templates (htmlHeader, branding...) the context variables 'content' and 'def' refer to the page content and page def. Since version 4.5.x these variables refer to the area's content and definition. Thus code like

[#assign propertyXXX = model.XXX!]
...
[#assign propertyYYY = def.YYY!]

should be migrated to

[#assign pageModel = model.root!]
[#assign pageDef = pageModel.definition!]
...
[#assign propertyXXX = pageModel.XXX!]
...
[#assign propertyYYY = pageContent.YYY!]