Are you getting the error “open_basedir restriction in effect” in your PHP error logs?
If you do see a message like:
Warning: is_executable(): open_basedir restriction in effect.
File(/usr/bin/tar) is not within the allowed path(s)
then this indicates your problem could be resolved by changing your open_basedir paths; or, by copying or symlinking tar into one of the existing open_basedir paths, then changing the location in config.inc.php.
If you see another message in the error logs, that would indicate that something other than the open_basedir restriction is at play.