Hello Heidi, after reading your post, I just get alerted I also has same issue on my machine too . There are two subfolders where drag N drop do not work. I cannot drag an email from there and also cannot dop an email from other "normal" folders.
I am not sure what goes wrong (maybe I have moved such folders before); but I just successfully resolve this issue by creating 2 new folders under same parent folder, move all emails to them from these 2 affected sub-folders, delete all 2 affected folders and then rename such 2 new folders to the original names of these 2 affected folders. Then no such problem till now. Have luck!