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When clicking on Show in the backend, this error occurs when an item is disabled, instead it should show the preview of the page.
The Exception occurred after the update of the core system to version 6.2.5.
The Exception occurred after updating to 6.2.9. When editing a tx_cal_event (/typo3/alt_doc.php?edit[tx_cal_event]=edit) and using any of the "Save"-Links, the Error "Page Not Found (404) The requested page does not exist!" is shown and nothing is saved. The resulting URL is /typo3/alt_doc.php?&edit[tx_cal_event]=edit&returnUrl=dummy.php which might hold two hints: dummy.php and the missing key/value in the query string directly after the question mark.
SOLUTION A customer of mine had the same problem. The solution was, that the SysFolder had a Page TSconfig entry options.tx_cal_controller.pageIDForPlugin = 419 The page 419 had been deleted. As I removed that option, cal worked again. So be sure to check the page TSconfig for wrong entries
This exception will also be thrown if an extension is passing an empty argument to the pageRenderer functions addCssFile and addJsFooterFile (tested on TYPO3 7.6.6).