URL rewriter: Unknown locale document-type-locale

When creating a new rewrite rule and saving it, the following error occurs:
WARN [org.hippoecm.frontend.translation.LocaleProviderPlugin.getLocale():129] Unknown locale document-type-locale

When manually adding a hippotranslation:translated mixin to the /content/urlrewriter node and to an underlying folder (eg /content/urlrewriter/myrewriterules), the issue doesn't occur.

The issue can be reproduced with the Hippo archetype (7.8.7) and the URLRewriter plugin: 1.04.00.

For the contentroot I use /content/urlrewriter and I tried to create the rules as an admin.