Don't have permission access [file] on this server?

I have installed EasyReservation Premium. I also installed all the extra modules that come with the premium plugin. But when trying to activate the extra modules, I get the following error:

Forbidden
You don't have permission to access /wp-content/plugins/easyreservations/lib/core/activate.php on this server.

Do you have any clue why I get this error? I am able to deactive and activate other plugins.

The website I am having this issue with is: http://www.vakantie-appartement-zwitserland.nl/ (login with 1st / website )