I had this problem before installing any plugin, on a WP “clean installation”. Anyway I tried to deactivate all plugins, but this didn’solve the problem.
I also switched to default theme, but didn’t change anything (besides, the problem shows up in the admin panel, which shouldn’t be affected by theme problems, I think).
I’m wondering if it might be some hosting issue, but I checked that allow_url_fopen is on, so I don’t what else to look for.