Form submission results in 404 error

I have a form that, when submitted, it will give me a 404 page, but only when the textarea has an anchor tag with an attribute (including a simple <a href="stuff">). No idea how that makes a difference to the routing of the system.

(You can see I’m not using any of the reserved names for WordPress, which seems to be the most common form/404 issue. In fact, I can make the names something like ‘lkskld’ and it still won’t work.)

<form action="messages/?to=0" method="post">
	<label>Send message to: <select name="msg-to">
		<option value="3">Test1</option>
	</select></label><br>
	<textarea cols="100" rows="7" name="send-msg">Here's a message with an <a href="go/here/">anchor tag</a>.</textarea><br>
	<input type="submit" value="Send message" name="snd-msg">
</form>