answer content is empty

Have you tried to disable all the plugin to check this issue, I think the plugin conflicting with another plugin. 
After disabling all the plugin to check if you still face their issue, you can send me username & password of your site for further checking. 

Is there a way to completely disable the visual editor and use a simple text box?

You can add the following code to the functions.php file:

add_filter( 'user_can_richedit' , '__return_false', 50 );