ArcGIS Pro 1.0 fails to sign-in, typically with the following error message:
"ArcGIS Pro cannot be started because it was unable to initialize license. Please contact your organization's administrator to check if your licensing portal has been configured correctly."
This may be caused by the presence of an environment variable used with a current or former install of ArcGIS Desktop. The environment variable is ESRI_SOFTWARE_CLASS.
If the ESRI_SOFTWARE_CLASS environment variable is not needed when running ArcGIS Desktop, then it should be removed. To check for this environment variable (Windows 7 example):
1. Click Start, right-click Computer and click Properties.
2. Click Advanced System Settings.
3. In the System Properties dialog, Advanced tab, click the Environment Variables button.
4. Check for the presence of the environment variable ESRI_SOFTWARE_CLASS under both User and System variables. If found, highlight the variable and select Delete.
In the case where the environment variable ESRI_SOFTWARE_CLASS is needed for use of ArcGIS Desktop, then the batch file accessed in the following link can be used to enable the use of ArcGIS Pro 1.0.
Extract the contents of the file startArcGISPro.zip and run startArcGISPro.bat.
This issue will be fixed in ArcGIS Pro 1.1.