When opening the CS, sometimes the creature/item etc.. tabs will fail to load.. why does this happen?

Probably because more-recent-than-CS Windows versions changed the way user interface auto-refreshment works.
Usually just dragging/resizing the windows helps. You can notice this expecially with the region colors map.
[EDIT]more or less, the CS still works reasonably well even in Windows 10, as long as it has administrator privileges you can even drag & drop .nif files and see them in preview. The one thing Microsoft really dropped is sadly support for CS help file format.