I've newly installed ArcGIS Pro 2.4 and created a bunch of layouts from existing MXDs for testing purposes. So far the product is useless to me as the final result of my workflows is always to generate a PDF and Pro has yet to succeed at this. I am using Share -> Layout Export. The output I am getting is a file where every byte in the file is zero. It does the same thing for TIFF and JPEG exports. I'm only trying to export letter-size layouts at 100 dpi. I have 16GB RAM and 50GB free disk space. I'm running Windows 7 Ultimate. Pro is not reporting an errors.
Another clue is that when I make a second attempt to export to the same file as an earlier export Pro often complains that it cannot write to the file. However, I have no difficulty deleting the file from the Windows shell, so there does not seem to be another process blocking the file.