I just downloaded and untarred openacs-5.1.0.tar.gz without error. Are you using the same environment that you used previously to untar? Could the file get corrupted as you download it? The MD5 for the tarball I got is c749a4d57d39cb4f1f4529be1922cbb6. You can get this via "md5sum openacs-5.1.0.tar.gz".
