Thanks for writing in! I went ahead and tested the subscription process on my end and was able to successfully use the sign up form in both Chrome and Firefox. Can you confirm the step-by-step process you're using that generates the 404 page on the subscribe process?
Currently the unsubscribe link does appear to be resulting in a 404 page. In taking a look at the URL it's pointing to, it appears that something may be incorrect about the way that link is being generated, which is causing that incorrect link to happen. To confirm, what version of the plugin are you currently using? Also, have any changes been made to the plugin's code itself?
One option you may consider is to try disconnecting, then reconneting the plugin. To do that, log into your WordPress admin area, then go to Plugins > Installed Plugins > MailChimp > Settings. From there, click the log out button, which will disconnect your MailChimp account with the plugin. Then, reconnect your account and re-select your list. After making these changes, then try using both the subscribe and unsubscribe process again, to see if the issue's been resolved.
If you have any further questions, ket us know!