ADE 4.5.1 does not respond when downloading or transferring books

I'm on Windows 10 and had no trouble with ADE until last month. Now after downloading an ascm file, I get a "(not responding)" message when I try to open the file. After I close and reopen the program, the book is there. Then I try to copy the book to my Nook and again, I get the "(not responding)" message. The book doesn't transfer to my device.

I've unauthorized and reauthorized ADE, uninstalled and reinstalled various versions of ADE, unplugged and plugged in the device, run as administrator, and still, the problems persist. Because it doesn't open the original downloaded file properly, I expect the problem is with ADE and not my device. What else can I try? Thanks.