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>
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And if I can make a recommendation, use the search here -- it's your friend, this issue has already been discussed ad nauseum; your your answer was two clicks away.