In the meantime the category page I spoke about seems to be gone from the SERPs. It's replaced by product pages belonging to that category (1 product page when settings are 10 results, 4 product pages when setting to 100 results).
And now the same effect I've described in my first post seems to start with other category pages.
My category pages contain unique content, are not overly SEOed and were visited often. They had low bounce rates, too. And they ranked more or less stable for their category keywords in Google for years.
Anybody observes similar effects?
And more important: Anybody an idea what to do?