[403] Cannot activate or deactivate plugins

I’m trying to install Sucuri Scan / Secure WordPress but get this instead:
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Access to the webpage was denied

You are not authorized to access the webpage at [site]. You may need to sign in.

HTTP Error 403 (Forbidden): The server refused to fulfill the request.
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I haven’t installed anything lately, and I’m really concerned. I had Exploit Scanner installed but I couldn’t access it (gave me the same message when I clicked in Exploit Scanner’s settings). The other stuff seems to be OK. Anyone encounter something like this in the last? I’m able to FTP the plugins into the plugin folder but I can’t activate it. And if I activate it through the database, I probably won’t be able to access the plugin setting page. Please help!