Could you please make sure that the original DNGs are not missing and are located through Lightroom?
Also, you might want to check for the permissions of the export location.
Thank you so much for your reply. I have checked for the originals and they are on the drive, no problems. Then I selected one picture in the LR library and clicked on the Export button. I went through all the settings and then thought that "Add to this catalog" should probably be ticked, which I did and whalla, my problem was solved.
Thanks for the update.
I believe "Add to this catalog" might import the same exported image back to the same Lightroom Catalog so, you might want to test it with "Do nothing" or "show in Explorer" if you do not want the same image to be imported back to your catalog.
Sorry yes, I thought that was the fault but when I looked in the export folder, there was nothing. So I had to adjust the permissions as you rightly suggested.
Once again, thanks for the tip.
Glad to help.