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 is /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.