Recently I had a client report that they were receiving this error message when deleting a sub task from a SharePoint 2013 Task list:
The list had been in place for 2 months. There were no reported issues previously, and there were no changes to the list settings. I was able to reproduce the issue by clicking the ellipses, but not when clicking the check box then using the Ribbon Row to delete the item, which I found a little bit odd.
After some poking around I noticed the URL when I deleted the task items using the ellipses contained owssvr.dll and NextUsing. See below url:
Fiddler to the rescue…basically what was happening behind the scenes is the call was successfully deleting the task item then immediately performing a GET request on the same URL instead of the default view. Then I recalled one of our other developers having similar experience with “wonky” POST/GET behavior when the Minimal Download Strategy feature was turned on…sure enough the client had enabled the Minimal Download Strategy feature and as soon as I deactivated the feature everything was working as expected.