Site links go to 404 after activating e-newsletter

I installed the e-Newsletter plugin via the dashboard on a Multisite install , and after activating it I cannot access any page in the admin area or even the site itself. All pages go to 404

My Pretty Permalink is set to /%postname%/.

There are only 2 other non wpmudev plugins on this site (assuming that the wpmudev plugins should not cause this behaviour), and these are:
1- Wordpress social invites:
2- UserPro:

Your help is appreciated.

Thanks and regards,