Internal links (anchors) to h1 stopped working

The internal links (anchors) to h1 headings recently stopped working in my book. To be clear, the following link does not work (even though it used to work perfectly well):

<a href="#anchor">click</a>
<h1 id="anchor">Heading 1</h1>

This also throws errors when I export the book to EPUB. Looking at the EPUB code, it seems that the value in the id attribute (i.e. “anchor” in the example above) is moved to a class attribute, so in the EPUB files I would see something along the following lines:

<h1 id="chapter-3206-section-1" class="section-header  anchor">

and then I get a “Fragment identifier is not defined” error in the export.

Has anyone else encountered this error recently?
Thanks a lot

This problem disappeared today. Sorry for the inconvenience