domain mapping issue when login to mapped domain

I've setup domain mapping plugin, and added custom domain successfully.
However I cannot login via custom domain, I got this error message

Error submitting form!<br />
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JavaScript and cookies are enabled.
For Administration mapping option, I've tried both "mapped domain" and "domain entered by the user" but they produce the same error. Can you tell me what the different between these two options?

Also when I set "original domain" used for administrator area, I can login but it uses mysite.originaldomain.com . I want to use custom domain for administrator area. Is that possible? Thanks.