This question is identical to [Resaving blog entries keeps failing][1] except that I'm not finding any [error] or [warning] in my log entries. 

I have changed my entry URL format for the section named Events from

    {mainCategory.last().uri}/{slug}
to

    {mainCategory.last().uri}/_entry

as described here: [Is it possible to view 'Expired' entrys on the front end?][2] and when I hit save on the section page, then it immediately gives the failed task: Resaving Events entries. Any thoughts?

UPDATE

OK, I did find this error:
2016/12/18 02:26:41 [error] [application] Encountered an error running task 422 (ResaveElements), step 2 of 197: Encountered the following validation errors when trying to save entry element "Transforming Work" with the ID "19":
 - URI "fort-collins/weekly/buddhist-meditation-classes/_entry" has already been taken.

I'm not sure quite what it means. There's only one entry named Transforming Work.

  [1]: http://craftcms.stackexchange.com/questions/9162/resaving-blog-entries-keeps-failing
  [2]: http://craftcms.stackexchange.com/questions/8226/is-it-possible-to-view-expired-entrys-on-the-front-end