If I simply try to access default shop page or display archives by product type (eg. https://test.antikvarijatknjiga.hr/?post_type=product), no results are displayed unless at least one of the faucets is selected. Also, selecting All results faucets doesn’t do anything.
The site currently looks ugly as all the customisations are turned off all the plugins except woocommerce with unmodified storefront theme and wpsolr, but it is functional.
The same template is displayed as default shop page (linked from empty cart for example).
All my products are simple products. Check out the same link right now with WPSolr plugin disabled.
The pages that are empty probably have some posts with some data missing (book author for example) which are rendered as empty strings without WPSolr, but when WPSolr is active, they end up breaking the loop.
I’ll do some debugging at your get_posts override and see what is specific about this data and get back to you.