I've used the plugin a lot before and never had this issue. As soon as it installs and I click on "activate plugin" I have no access to admin panel. it's as if the plugin already assumed a different URL, but I don't even get to see the check list and the normal admin/longin hiden URLs don't work.
This is the same host I've always used and it's only happening in this site. As soon as I activate, it goes to 404 error.