The remote host is missing an update as announced via advisory SSA:2004-124-01.

https://secure1.securityspace.com/smysecure/catid.html?in=SSA:2004-124-01

New rsync packages are available for Slackware 8.1, 9.0, 9.1, and -current to fix a security issue. When running an rsync server without the chroot option it is possible for an attacker to write outside of the allowed directory. Any sites running rsync in that mode should upgrade right away (and should probably look into using the chroot option as well). More details about this issue may be found in the Common Vulnerabilities and Exposures (CVE) database: http://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2004-0426