I select a number of images and use the Export dialog, exporting to JPEG. After the export is finished I get a dialog box saying "Some export operations were not performed. The file could not be opened ()", and then lists the file names of the images that were not exported.
If I delete the exported JPEGs and try again a different subset of the selected images gets exported. On average around 20 out of 70 selected images don't get exported.
Watching the directory where the images are being exported with a file viewer while the export is in progress I can see that the images that are later shown in the "not exported" dialog box list are in fact being created in the target directory but are quickly deleted.
I have had one example of exporting of a single image failing with exactly the same error message.
I have never had this problem with version 5. I just ran an export of 126 images using Lightroom 5.6 which completed successfully, as has always been the case with version 5 and before that with version 4.