youre going to need to disable mod_security.
if you do not have an .htaccess in your wp-admin/ directory:
create a text file on your desktop:
put the following inside it:
<IfModule mod_security.c>
SecFilterInheritance Off
</IfModule>
name it whatever you like and save the file.
Upload the file to your wp-admin directory.
Rename the uploaded file to .htaccess (with the .)
If you do have an .htaccess in your wp-admin/ directory:
Edit it and add:
<IfModule mod_security.c>
SecFilterInheritance Off
</IfModule>
--
there are cases where that might not work -- for instance, mod_security was recently changed so you might need a different little snippit of code. if this doesnt work, just post back.
and be careful with your edits, obviously.