[Plugin: CTForm7] Problem after activating it

I recenly installed the Contact Form 7 and I’ve the strange problem that when I activate it, all the plugins dissapear from the list, even CT Forms 7 (so I have to go to the server to delete it). Only 2 of 10 plugins are visible, the rest just vanish.

Additonal info: I’ve a blocked FTP server, so every time I need to upgrade any plugin, I’ve to enter user and password.

Thanks!