If you see a login error similar to the following:
You are probably using a version of the Intel XDK that needs to be updated. Versions of the XDK prior to and including 3400 are not able to log into the Intel XDK backend services. This is due to changes that were made to maintain a higher level of security for your accounts.
The version of the XDK that you are using is displayed in the upper right corner of the Intel XDK window (usually as a four digit number).