this error occurs in very rare cases. We've created a repair mechanism for it in our chrome extension:
1. Download Google Chrome (http://www.google.com/chrome) and install the Boxcryptor extension in it (https://chrome.google.com/webstore/detail/boxcryptor/ijmndaodmdjamfepoijpolhjddgfgmme?hl=en-US).
2. In the extension, click on the Boxcryptor icon on the upper right, and log into your account.
3. After you're successfully logged in, right-click on the icon and choose "Options".
4. In the appearing window, change your password (you can change it back afterwards). After you changed your password, your keys are encrypted with your new password, which might take a few seconds (depending on how many keys you've collected through sharing files with Boxcryptor over time). Afterwards, you should be able to log into the Boxcryptor app for Windows again. You can of course deinstall Chrome and/or the extension again if you whish to do so.
Since this error happens quite seldom, we couldn't find the source of the problem yet. Would you mind telling me which of our apps you're using (e.g. Windows app, Android, Chrome extension, ...)? This could help us in the search for the cause of the issue.