Template "page.sequence" in fo/component.xsl doesn't check for empty content and hence creates empty fo:page-sequence/fo:flow, which is invalid !
In my case it happened with all tables having attribute tocentry=0 and nothing else to list.
I checked in an enhancement that checks the $content param before generating the page-sequence.
I'm collecting input from different sources in our product, some are XML, some are plain text and I convert them into DocBook fragments. Sometimes, they are 'empty' and I run into the same error message as above. In those cases it is correct and I have to fix it by not supplying the empty container or add some content like 'NO CONTENT AVAILABLE'. But I need to find out, where this happens and working from issue to issue is sometimes quite annoying. I would like to have a default content (highly visible in the result, like unknown XML tags) to indicate missing content, instead of getting an FO error message. - No I don't want to make a customization for this :-)