How to resolve the error about 'A partially-completed form was found'?
Since "deleting partially filled nid in webform submissions table" (as in your question) seems to not solve your issue, you may want to have a look at comment nr 2 in issue 2560907, which states:


  A webform node is being displayed on that page and there is a draft submission for the current user. You don't have to find the id -- the form will be displayed, unless theming or another module is preventing it.