I was trying to establish circumstances first.
The first step I’d take would be to make a copy of the project and delete the Intermediate and Saved folders. That might just do it. If it does not help, see if you can figure out whether it happens for one widget only or all of them. Does it happen to newly created widgets, too?
Ctrl+A should select either all folders in the Content Browser or all items in a select folder. This should also apply to widget Hierarchy tab. Can you tab out and ctrl+a items in windows folders or is it only UE4 that’s misbehaving?