When a product has an attachment the link in the frontend is .../item/download/1251_2b7f58ceee4aba56d1c5f86d3f0c18fb
If you click on the link you get a white page with the error that the file doesn't exist, although it's saved in the database and on the server in /media/k2/attachments. However the link does work when you remove the hash code at the end: .../item/download/1251
This only happens with files that were saved in the database before the site moved to a different server.
Attachments that are uploaded today do work.
So why does the old hashes don't work and how to generate new hashes?

As mentioned in www.joomlaworks.net/forum/k2-en/47194-k2-attachments-return-404-error-file-does-not-exist I commented out the hash check. This is a temporary fix untill an update. I hope there is a more clean fix?