I use K2 for creating a catalog of companies and products.
in the description of the product and company I use an extra fields, including the field of website.
after upgrading Joomla from 1.7.4 to 2.5.3 and upgrading K2 from 2.5.4 to 2.5.5 I got a problem with Extra Fields.

For example, in the catalog, there are two products from one manufacturer
in the description of a first product website in an extra field is displayed correctly
www.smart-edu.com/instrumenty-razrabotki-uchebnyh-kursov/1738-competentumavtor.html
and in another product instead of the website displays the text [{"name":"","value":"http:\/\/","target":"new"}]
www.smart-edu.com/inctrumenty-poderzhki-soveschaniy-konferentsiy-virtualnyh-klassov/1852-competentumwebinar.html
in the administration panel Text, URL and attribute "Open in new window" are the same

what could be cause of the issue?

interesting observation:
If the text field "Fax" is empty
then I get an incorrect significance of the extra field "Website"

I have exactly the same issue.

I noted that all is ok with extra fields if all fields have data. But if a item has some empty fields (no values), the last fields values (same number of empty) will get values like [{"name":null,"value":"","target":null}] for text fields and [{"name":"","value":"http:\/\/","target":"new"}] for a link field.

It seems that json decoder doesn't work properly. Is it a K2 bug?

Yes, this does is a bug. The issue was fixed and will come working properly with K2 2.5.6 version: code.google.com/p/getk2/issues/detail?id=197

Thanks to DEV team.