Hahaaa. If all else fails check the sticky. I’ve been banging my head against my desk for ages, and there it is.
In case a google search (which is how I found out the solution) brings you to this entry then follow the below –
6. mod_security might be causing problems. Disable it to see if that is the problem. To do this, make an .htaccess file in your wp-admin directory. Add this to it:
<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>
That will disable mod_security for the upload file receiving code in the wp-admin directory.
FOR THE NONE TECHY – what this means is create a notepad .txt file on your desktop, call it something like access.htaccess, copy/paste the above and stick it in your public_html folder (or root directory).
I had to figure it out myself, so hopefully I’ll save some poor soul the trouble if you find your way. ??