Polycom CX700 cannot sign in after upgrade firmware to 4.0.7577.4066

Hi,

We have one CX700 phone with old firmware version -3.5. We tried to upgrade the firmware to the latest version 4.0.7577.4451, but failed. Then we refer to Jeff's blog (http://blog.schertz.name/2012/08/lync-phone-edition-cu6-upgrade-issues/), and managed to upgrade to CU5 4.0.7577.4066 successfully. However, we found that after upgrade to 4.0.7577.4066, we cannot sign in from CX700 anymore, please find  the error message below which is shows up on the phone:

Cannot sign in. Please verify your sign-in address domain\username, and password and try again. Please verify that the domain entered USERNAME is correct.

And we cannot upgrade the firmware from 4066 to 4451 without sign in.... (very inconvenient design...)

Any ideas?

Thanks a lot!