If you try from top menu category t-shirt and from openswatch filtering funny the first 20 products in the first page is ok in page 2 in pagination the products is wrong we see at the pagination in page 2 other products with other selected attributes and not the funny and this is a sample to understand in the most of categories in openswatch we have the same problem.
Here is the results we see in the page http://mplouzaki4u.gr.dedi3709.your-server.de/shop?product_cat=t-shirt&source_id=129&source_tax=product_cat&filter_%CF%83%CF%87%CE%B5%CE%B4%CE%B9%CE%B1=funny
Best regards