I had exactly the same problem on my older web hosting.
It is not a problem of Seo4Smf but at your web hosting.
Safe_mode may be activated on your server - Your server is too secure. He forbids advanced Php actions.
Try to ask to your web hosting to change option safe_mode Php or change web hosting, like me.
Good luck.
thx.
Maybe some other suggestions, how to fix this?
Just want to tell, return to topic are work on forum without SEO4SMF. Is problem in .htaccess file?