404 error on search result page

The issue:
Since a while ago, every now and them the search results page returns error 404 and is not working. Only after I go to Admin Panel -> Settings -> Permalinks, and save the setting – it comes back. but only for a short while.

I use S&F as my main search engine, so search.php template file contains do_shortcode('[searchandfilter id="'.$formID.'"]'). The search form setting are:
– Display results method: As an archive
– Use a custom template for results: V
– Enter the filename of the custom template: search.php
– Set a slug: search.
It has been working perfectly until just a week ago, maybe there was an update, I can’t recall.

Another important thing – the site’s server is Nginx. so no .htaccess as far as I understand.

Thanks,
Itamar