This happens when the application opens up an Activity Note on a much, much larger size than what your screen supports. What you are seeing is a highly zoomed in view of a part of the screen, hence it looks all white (or sometimes all yellow).
This is easily fixed by deleting the file that stores the last window sizes.
That file is called “crm.user” and depending on your version of Legrand CRM it is found either in ‘My Documents\Legrand Software’ or in ‘My Documents\Legrand CRM’
Simply delete that crm.user file and start Legrand CRM again.
Below is a screenshot of the location of that file in ‘My Documents\Legrand CRM’