WordPress Cookie Issues

Hello All,
I working on a site with a heavy emphasis on the user features of WordPress. Each user account is tied to additional custom attributes so we have a significantly larger WordPress User Account base than “normal” WordPress sites. It is currently in the hundreds and growing daily.

The issue that we are having centers around WordPress Cookies for user login. I have had several people contact me saying that they cannot log into the system, even after running through the forgotten password routine. The solution has been, in every case, to have them delete their browser cookies.

I don’t want to have to force a user to delete their cookies every time this happens. Is there a way to invalidate a cookie, or force the system on a per user basis to create a new one. Do you have any ideas as to the cause or possible solution to this problem?

We are also running the site integrated with the Simple:Press forum system. The user accounts are shared across both pieces. Thanks!