If I use the text "insert into" anywhere in a post, MT [version 3.11] completely gives up the ghost and reports a 500 Internal Server error, not even allowing the entry preview, let alone saving it.

It even does it with a post that contains nothing but the text "insert into".

I'm not kidding!

Perhaps it's worth pointing out that INSERT INTO is a mySQL command. Still not sure why anything that's in an entry field would be parsed as a command instead of just saved in the database.

DELETE FROM also causes a 500 error.

I was able to create an entry with both of those in it without incident.

Are you using any plugins that might cause this?

I'd not thought of that. Unfortunately, no joy. I deleted all of my plugins and still had the same problem.

I've discovered it also affects comments on my site: you can't submit [or even preview] a comment which contains INSERT INTO or DELETE FROM.

Hmm. I can enter those statements into a comment too.

Do you have background tasks enabled?

What database option are you using?

Yes, I have background tasks enabled, but disabling them didn't fix it.

I'm using the mySQL option, which is why I thought there might be something to do with the fact these strings are SQL commands.

Okay, I had a brainwave and it seems that this problem is common to MT weblogs that share the same host as me. I'm contacting their support desk now to see if they can help!

According to this Bug report this issue is due to a configuration issue of the ModSecurity software.

Get in touch with your ISP.