Filters return 0 products till refresh of page

Not everytime, but occassionally when clicking on any filter, 0 results are returned even if filter shows products attached to it. When clicking another filter or clicking Filter button, nothing happens. Page needs to be refreshed and then everything works. Sometimes it happens in the middle of configuration.
Please check here: http://testapp.pcguru.lt/vietiniu-tiekeju-sandeliai/
(tried both with http and https)