Magento 2 product page thumbnail stretched and blurry

We are running Magento 2 for quite some time now and all was fine for a long time.

Some time ago (we can’t really pinpoint when this happened) all the thumbnails on the product page went blurry. It seems like the images are resized incorrectly and stretched a lot to show them in the frame.

I added a screenshot to make it clear.
Blurry thumbnails Magento 2 Product page

How can i fix this problem?