When the survey link is clicked, it just displays a blank page in the portal
It is most probably caused due to the skipped London upgrades in the script include "SPSurveyAPI"
Starting from London, the "one click survey" feature has been introduced, which when enabled in the survey definition, it would skip the survey intro page in the portal.
So the widget code is updated to check if the survey has "one click survey" enabled and it will skip/load the intro page accordingly.
The widget gets that information from the script include "SPSurveyAPI".
But the script include "SPSurveyAPI" has been customized and so it has skipped the London upgrades. Therefore, it does not provide the widgets with the information regarding the "one click survey".
So the widget code is not able to determine which page to show and therefore it displays the blank page.
To resolve the issue, please revert the script include "SPSurveyAPI" to the latest London version and then apply any customizations on top of it, if you like.
1. Please refer to this documentation for more information on "one click survey"