I was having the same problem. Removing -v didn’t work for me. My use-case is slightly different in that I’m going from source (EXT4) to ExFAT. The issue for me was that rsync was attempting to preserve device files and permissions, which ExFAT doesn’t support. I was using the -hrltDvaP switches. The -D and -a switches seemed to be my problem. The -a switch translates to -rlptgoD (no -H,-A,-X). The -p, -g, and -o switches seemed to be my root cause as rsync was barfing on one or all of those during runtime. Removing -a and specifying -Prltvc switches explicitly is working for me.
bkupcmd="nice -n$nicelevel /usr/bin/rsync -Prltvc --exclude-from=/var/tmp/ignorelist "