Hi,
create a little PHP script that just contains
<?PHP
echo $_SERVER['DOCUMENT_ROOT'];
?>
This should print the full path to the DOCUMENT_ROOT directory on your server. This path should contain one of the directories you posted.
Try to remove the last directory of that path and use that as path to the error log file.
Is it possible for you to create another folder in your directory on the shared server ?
Examples:
The PHP script prints e.g.
/www/hosts/blabla/public_html/........
So you know that /www/hosts/blabla is your root directory on the shared server.
Try to use /www/host/blabla/phperrors.txt in your .htaccess file or create a directory like
/www/hosts/blabla/phplogs and use
/www/hosts/blabla/phplogs/phperrors.txt
whatever you like more. You might need to chmod that directory.
Thomas