I need the solution too, I have the same problem! Could you please send it to me via email to This email address is being protected from spambots. You need JavaScript enabled to view it.?
Thanks in advance!

I disable SEF... but that is not a soloution!

Make sure it is not a template issue by using the system template.
Simply add ?template=system to your URL and check if it works with the default template.

If it does you should download K2's latest verion. Open item.php and transfer the newer attachments code to your template.
This post will help you locate the files you need to edit.

nuevvo.com/blog/item/84-k2-inheritance-sub-templating

I solve the problem, disabling only the SEF for K2..

Components--> K2--> Parametrs --> Advanced Tab --> And
Enforce SEF URL character replacement NO

Thanks!

I have same problem. If i disable joomla sef, к2 attachments link works fine. J2524, K2 -268