Thanks for reaching out to us for your concerns.
Can you please let me know the exact version number of Adobe Acrobat DC that your are working on ?
I Tried this at my end and Drag drop in Page Thumbnails panel in a pdf / across pdfs is working fine for me.
Also i would request you to kindly share with me a screen recording video of the steps that you are trying to do here ?
You can send me that at email@example.com
An error occurred while saving the commentGary Brazzell commented
Just developed this problem today, out of the blue. Zooming out does not help. It simply won't let me rearrange pages. The main way I use PDF is to create a file by combining different files. In the first screen, I can usually drag and drop thumbnails. Now I can't drag thumbnails at all, not from any of three panels where I would usually do this. Also, my tools are all messed up. The main tool I ever use is the zoom / magnification, and it's gone.