http error on image upload

I’ve tried different htaccess fixes lots of people say work like

AddType x-mapp-php5 .php

and

<FilesMatch “(async-upload\.php|wp-cron\.php|xmlrpc\.php)$”>
Satisfy Any
Order allow,deny
Allow from all
Deny from none
</FilesMatch>

and

<IfModule mod_security.c>
SecFilterEngine Off
SecFilterScanPOST Off
</IfModule>

I’ve tried to find the line referenced in https://vali.us/2010/how-to-fix-http-error-in-wordpress-2-9.html

but that was 2.9 and I guess the line has been changed since then.

It’s not that ALL images are unable to upload. I’ve tried a 25.4kb jpg of 160×159 and that uploaded just fine. I’m really at wits end here.