Error popup "GuiXT license file missing": An error popup is shown with the above title and text "Please specify the location of the GuiXT license file in GuiXT profile". What could be the reason?

If you only use GuiXT you don't require a license key or a license file. Please check in your GuiXT configuration that you only have  GuiXT selected under "installed components" (component GuiXT, in guixt.ini). If you are using a new GuiXT version (as of 2015 Q4 2)  GuiXT only will be selected automatically if no valid license key and license file are found.

If you have licensed other components (InputAssistant, Viewer, Controls) a license file reference is required in GuiXT profile under the following conditions

SAP GUI 7.40

or 

SAP GUI 7.30, GuiXT before 2014 Q3 2, as of January 1st, 2015

The license files are sent to you automatically by Synactive. If you haven't yet received them please contact us.

For details see http://www.synactive.com/docu_e/doc_licensefile.html