Error message continues: ("The file is not found") when calling ShellExecuteExW from AgWorkspace.shellExecute. I have Win 7 64-bit Lightroom v. 5.6. The error will occur when I am exporting a file. After clicking OK the file is saved despite of message.
In the Post-Processing section of your Export dialog, the "After Export:" field is blank (which is invalid, and causes the error). Simply use the dropdown to select a valid option such as "Do Nothing" and the error will stop.