I personnally tested my urls on the claim form and it worked.
If you are getting a 404 error it could be that your url is made up.
All instawallet urls that have received transactions are in the database.
My URL is not made up and I can't acces to the claim process.

Okay I got it now. You can't use your bookmarks. You need to go to the instawallet.org and paste /w/xxxxxxxxxxxxxxxxxxxxx to the url.
Possible shill alert.