It looks like there is an issue with Dropbear and setting the $PATH environment variable in TC 6.x.
In the version for TC 5.x, an SSH connection set
PATH=/usr/local/bin:/usr/bin:/bin
In the version for TC 6.x, the SSH connection only sees
PATH=/usr/bin:/bin
TC 5.x and 6.x both load several extensions into /usr/local/bin, and because dropbear now sets PATH differently, rsync will fail to execute without passing the --rsync-path=/urs/local/bin/rsync flag.