Unity 5.1.3 does not let me sign in. Why?

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I have tried logging into unity 5.1.3f1 for at least 20-30 times with no results. Note that my email and password are correct and I have internet connection.

Sometimes this thing happens , it happen with me also i uninstall the unity and install it again then i did not get the error again after reinstalling the unity it is working perfectly