The Android version of the Dropbox interface (on the Kindle, anyway) lets you create a password like this:
Come to find out that, although the web-based Dropbox interface doesn’t reject that password, it kvetches that your userid and password don’t match. Yes, even if you cut-and-paste from a text file copied through the USB link.
Fortunately, the web interface has a password reset mechanism that’s missing from the Kindle app.
Little Bobby Tables rides again!