e-Newsletter plugin freezes all Admin pages

I installed the eNewsletter plugin v2.7 on my eHangar.com site which is running WP3.9.

When I activate the eNewsletter plugin, all the Admin pages, including the network admin, become inaccessible and only display a white screen.

The site's webpages, however, are all working fine. It's just the admin pages that are affected.

I am unable to deactivate the plugin in the normal way to get the admin pages back again. I have to FTP into the site and delete the eNewsletter plugin and then the admin pages show up normally again. I found that by renaming the file e-newsletter.php to something like e-newsletter.bak, I can also get the admin pages back again.

Can you please help me here as I want to send a newsletter to my site members as soon as possible.

PS: Why isn't the eNewsletter plugin listed in the dropdown list above?