I have the exact same problem with my Rhino 5 which I fixed with a toolbar reset. However, my T-Splines toolbar icons are still missing! No one from the T-Splines forum has a fix for this either. I think it may be a Windows 10 upgrade conflict. I’ve tried rolling back to Windows 8.1, but I think the damage has been done. Since Autodesk purchased t-Splines, their support has been non-existent! Frustrating!