It is not a problem of both running together.
The problem is a combination of :
1) plugins assumption about where the wp files are:
The message is about line 11, but problem caused by
line 7 of wp-simpleviewer\config.php :
$wp_path = preg_split('/(?=((\\\|\/)wp-content)).*/', dirname(__file__));
2) the site in question has their wp-content relocated: it is not under the WP_SITEURL folder. See http://codex.wordpress.org/Editing_wp-c … wp-content
So line 11 fails to load the bootstrap.
I dumped the new plugin down to my local test site and all is fine if one has 'normal' wp structure. So I guess for now, anyone with wp relocated will have to stick with old version.
Hope that helps