I couldn't log in, so I thought my password might be a problem. I changed my Minecraft password (removed symbols) and I was able to log in.
You should ensure that all symbols that can be produced by a standard US keyboard are supported, e.g.
`~!@#$%^&*()-_=+[{]}\|;:'",<.>/?
Ideally, just encode the text for POST-ing and allow any password or passphrase. Minecraft is pretty accommodating in this regard.
Thanks!
Justin