Hockeyfreak solved this for me!
My htaccess file didn’t have the code he suggested at all, commented out or otherwise – I simply pasted in the code after whatever was already there and it works! I’ve added it to all my WordPress sites!
I would say this might be a host by host problem and you might have to find your own fix, but luckily so far all the sites with problems are sites I resell hosting for with the same host.
Again, this is what I pasted to fix my http error:
<IfModule mod_security.c>
<Files async-upload.php>
SecFilterEngine Off
SecFilterScanPOST Off
</Files>
</IfModule>
Thanks!