I also had that problem and emailed stardock support. Here is what they said:
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Please make sure that you are running the most recent version, which
is located at:
http://storage.stardock.com/files/stardockcentralrc1.exe
Then, close and reopen it to run the auto-update feature to make sure
that it is the most current version.
Next, go to Tools>edit Stardock.net account
Make sure that the password field is filled in with the correct
password. If it is not filled in, or you are not sure if it is
correct, please go to:
https://www.wincustomize.com/index.asp?cmd=SENDPW and use the lookup
to have your password emailed to you.
Other reasons why it won't login:
1) you are using a different email address than what you registered
with.
2) the nick that you are trying to register with is already taken and
SDC is giving you an incorrect error.
3) you are using a firewall, proxy or ad blocker that is interfering.
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Worked like a charm for me..