Running the Updater or autoinstaller, etc., on RHEL 5.x may fail with:

This build is not compatible with current OS: Current OS is Linux RedHat el5.1 i686
but this build for build for Linux RedHat el5 i386 (/root/psa/PSA_8.2.1/plesk-8.2.1-rhel5-i386.inf3)
ERROR: The build version you tried to install does not suit your operating system. 

The problem is that Plesk is looking for "release 5" in /etc/redhat-release and the "release 5.1" throws it off.

Solution: edit /etc/redhat-release and change "release 5.1" to "release 5" so it looks like:

[root@server1 ~]# cat /etc/redhat-release
Red Hat Enterprise Linux Server release 5 (Tikanga)

Yes, this is ridiculous. Remember to change /etc/redhat-release back to the way it was after performing the upgrade.