Unable to deselect thumbnails all of a sudden

We print construction plans and very often need to select specific pages. To do this, we have always opened the thumbnail panel and CTRL-clicked to select AND deselect the pages we need. We are now unable to deselect pages; CTRL-clicking on a selected page now has no effect but - for years and years - that's exactly how we deselected pages. Of course, CTRL-click works that way all throughout Windows.

Using the "organize pages" function is of little use because, even when enlarged, the thumbnails are too small to read the speicilaized page numbers which appear on construction plans.

This is occuring on all 5 employees' computers at my job. 

1) Is anyone else experiencing this?

2) Is there a solution?