I am on the ten-day trial and have processed a couple hundred images. In most cases I have had no problem exporting a jpeg or tif, and no trouble saving the final version of my work as an afphoto file, which I have been able to reopen to adjust.
However, with absolutely no change in my workflow or method, on occasion I am unable to export a tif, although I can save the file and can export a jpeg. If I export the image as a tif, the icon on my desktop is black, and if I open it, it is blank.
This is so random that I am unable to figure it out. If I close the image and start fresh, I get the same result. I can go to the next image I want to process and have absolutely no problem saving the file and exporting both a tif and jpeg.