[Plugin: Events Manager] #_EVENTIMAGE{x,y} 403 error timthumb.php

This topic was already discussed here and marked as resolved:
http://wordpress.org/support/topic/plugin-events-manager-_eventimagexy-doesnt-work

Error message for a normal visitor (no login) in Chrome:

403 Forbidden
You don’t have permission to access /wp-content/plugins/events-manager/includes/thumbnails/timthumb.php on this server.

However on my WP 3.6.1 with Customizr theme on shared hosting (Apache, php-fastcgi 5.4.16) usage of #_EVENTIMAGE{x,y} seems still a problem as:
a) commenting out in /events-manager/includes/thumbnails/timthumb.php “$base = $this->docRoot;” had no effect
b) “folder permission error of some sort” is a vague indication: like all other data in wp-content the dirs are 0755 and the files 0644. Owner is the FTP-User, changing it to the www-data user for timthumb.php had no effect on the error message. What could be wrong?
c) usage of <img src=”#_EVENTIMAGEURL” width=”100″ height=”90″ border=”0″> is a workarround, but no solution IMHO because if the editor is not capable to cut and resize images accordingly, this will lead to bad (uneven) layout.

I have noticed that when logged in as admin #_EVENTIMAGE{x,y} shows up fine (tested in Chrome and Firefox). So could this be related to the login status?

Thanks for your help.