@Lefteris, It was and it is set as NONE :(

Is there a link to your site?

yes darmolandia(dot)pl

You cannot have URLs like www.yoursite.com/xxxx for K2 items ( or any other extension in Joomla!). The only way to do this is to create menu links for each item or use a SEF extension like SH404SEF.

K2 Advanced SEF is designed to work when a resource ( a K2 item in your case ) is not resolved in any menu link ( there is no menu link to that item OR it's category ).
In that case instead of having menu links like component/k2/xxxx you have the option to either use the category alias or a custom alias to replace the "component/k2" part.

Ok, thank you very much for the information. I haven't found that.

Your answear finally resovles my doubts.

Thank you!

You are welcome.

I just loaded v2.7 on my staging site - why is there no option to remove Prefix for Category URLs?

Hello Joe,

A prefix is needed in order to work properly. You can remove the prefix entirely by mapping the category into a menu item.

Ok - Thanks for the info

Is this a limitation for all non com__content extensions?

I wouldn't know actually.
It depends on how extensions are built.