why drupal webform form setting shows mysql errors in dblog?
INSERT INTO {webform} (nid...


tries to insert the nid of the webform (got from auto-increment in the node table) in the {webform} table. So, every nid inserted in nid column of webform table should be unique. But it seems that the newly-created webform node got a nid that already exists in webform's nid column. 

So I guess there's something wrong. Find that nid in webform table, see if it exists in node table, that must be the trick.