Samsung: Backing up messages fails
I had the same problem for month, I updated to the last version of smart switch today and have reboot my s8 yesterday (I have almost 25% of free space on the phone)
I gave a new chance to the backup with smart switch today and, after being stucked for almost 2 hours on "backing up message" at 18%, it finally went to the next step of the backup!
So my advice is : patience (and use an usb 3 port!)