Sometimes hosting providers will implement mod_security (and not mention anything). Or else, mod_security was already on but a recent installation of a plugin or theme may have exposed this new problem for you.
You can try these four things…
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Add the following to your .htaccess file (add to .htaccess, don’t replace the other stuff already in it)
<IfModule mod_security.c> SecFilterEngine Off SecFilterScanPOST Off </IfModule>
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chmod all your folders (including plugin and theme folders) to 777 temporarily just to confirm that it’s really not a permissions issue with file owner permissions. If it is, then you can go from there (maybe your site isn’t really running as ‘myftpuser’.
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Deactivate your plugins, one by one, testing your post after deactivating each one. (See if a plugin is causing this).
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Does the content you’re testing have the word “from” in it? If so, can you post without using that word as a test?