Today I booted up my personal Confluence as usual, to find the header mysteriously broken:

The error in black-on-blue is:

$soyTemplateRendererHelper.getRenderedTemplateHtml("com.atlassian.auiplugin:aui-experimental-soy-templates", "aui.page.header.soy", $templateParameters)

Nothing unusual in the logs.

The fix

After the usual futzing around, I found this KB article describing a similar problem. In the UPM plugin (the URL isĀ /plugins/servlet/upm?source=side_nav_manage_addons by the way) under the 'System' plugins, there is indeed one module disabled:

The SQL fix from the KB article worked:

UPDATE BANDANA SET BANDANAVALUE='<map/>' WHERE BANDANAKEY='plugin.manager.state.Map'

If I ever find out why this happens I'll update this post.