I have been battling this problem as well and thought I’d add some info. One of my options page fields is a 'type' => 'wysiwyg' field. If the user adds any html field that generates quotes in the markup I get the 404. As long as the markup does not contain quotes it submits fine.
For example, centering text generates <p style="text-align: center;"> which blows out the page, but <h2> is just fine.
I don’t have a solution (yet) other than changing the field type.
