Featured image doesn't work

Hey Guys,

My featured image function has stopped working. I use Chrome 47.0.2526.106 m

Take a look, I cannot open the box for uploading pictures:
http://screencast-o-matic.com/watch/colvQghzUI
http://www.evernote.com/shard/s184/sh/8dc432d4-059f-4abc-998b-42a5d958470e/b1f46841302833df2fe01a37058c3063

I use PL translation, so “Obrazek wyróżniający” means “Featured image”.

I tried in different browser (FF 41.0.2)

Can you help me with that? Writing post without images in cook blog is pretty useless;-) Thanks in advance.

Best regards,
Olek