The setting works fine on our gallery:
https://www.photo.gallery/imagevue/?thum ... ,0,0,0,0,1
Click any folder in the gallery, and you will see it does not display any empty tooltip box. Then browse to the folder "Mother Earth" -> "Earth Animals", and you will see it loads and displays descriptions on thumbnail rollover.
I think perhaps you might have empty values in your descriptions ... Perhaps you have created descriptions, and later on deleted the description. This may possibly leave a blank space in the description. This would lead Imagevue to believe it is supposed to display the blank space. Try opening your descr.txt file in your appropriate gallery folder where descriptions are stored. Delete blank entries(Eg. entries with an image name and empty space behind).
Im not sure I would recommend using the descriptions on thumbnails because it is not bulletproof I must admit. The problem is when rolling over thumbnails, it has to make a call to the server every time to get the description, and then on rollout, it has to cancel the call.