I've actually solved this problem but wanted to add it here incase anyone else encounters the same thing and could use the solution. Also, I suspect that there is something the developers could do to correct this issue in future updates.
I was getting 404 errors on the submission of a form (though only intermittently). The problem turned out to be that I had labelled a field asking for the name of an emergency contact "name." It turns out that having the name value of a field as name="name" will cause 404 errors.
This would be a bad idea, for example:
<input style="text-align:left; margin:0;" type="text" id="fscf_email5" name="name" value="" />
The reason for the intermittent error was because the field was not required and not everyone was filling it out.
Discovering this issue took some time so I hope someone else will benefit from this - I've no idea why this produces 404 errors so if someone can explain the magic I would certainly be interested.