Hello! 

I have problem with ALT attribute - it shows unknown symbols (alt="|||||||||"). 
For instance, <img src="/media/k2/items/cache/ba1b7eb9b8ad142948e3b9dce300b4c6_M.jpg" alt="|||||||||">
It is from table k2_items - image_caption and image_credits fields. 

K2 generate these symbols

Can you just say, which PHP file is responsible for data insert in #_k2_items?

I'll try to fix it myself

I you are using override and you are talking about main picture on item page.

it should be /templates/your-template/html/com_k2/default/item.php > Line 199

I deleted title attribute from item.php

But problem is, that it insert ||||||||| in image_caption field in SQL

SQL query is not in item.php

Try to use the original item.php or use a static value there for alt. It may narrow down the cause of the problem