404 Error on Forum Submit in a Subfolder of Wordpress

I have an issue with a form throwing a 404 error on submission but only when certain words are in the body of the post.

Here is my form.

<form role="form"  action="edit-training-page.php?id=<?php echo $training_id; ?>" method="post" onSubmit="return confirm('Confirm Updating Training Page?');">
            <div class="form-group fl w300 mr">
                <label>Training Name</label>
                <input name="training_title" type="text" class="form-control"  placeholder="" required value="<?php echo $training_title ; ?>">
            </div>
            <div class="form-group fl w200 mr">
                <label>Short URL</label>
                <input name="training_url" type="text" class="form-control"  placeholder="" required value="<?php echo $training_url ; ?>">
            </div>
            <div class="form-group fl w100 mr">
                <label>Sort Order</label>
                <input name="training_order" type="text" class="form-control"  placeholder="" required value="<?php echo $training_order ; ?>">
            </div>
            <div class="form-group fl mr" style="width:100%;">
            <textarea name="traininfo" rows="20" cols="80" /><?php echo $traininfo ; ?></textarea>
            </div>
            <div class="clear mt " >
            <input type="hidden" name="training_id" value="<?php echo $training_id; ?>" />
            <button type="submit" name="update" class="btn btn-success fl mr"><i class="glyphicon glyphicon-edit"></i> Update Training Page</button>


        </form>

We have a sub-folder inside of WordPress for a dashboard system not related to WordPress. It is all php driven script. It is in a folder on the root of server, this form is for adding training content. I have been migrating training documentation from another location into this new system. It randomly throws a 404 on submission even though the URL is correct. For example today if the word "from" was in the textarea field it would give a wordpress 404 error screen on the submission. I have checked the reserved WordPress list in WordPress and none of the input fields are names that are reserved.

I tried excluding folder by editing the .htaccess file and changing the line to RewriteRule ./ /index.php [L] - No change.

The php code for processing the form is on the same page.

Any help would be appreciated.