I have had this bug a couple of times today. I am using Affinity Photo 1.7.1 and simply trying to Export a cropped version of a jpg. The error message says cannot export to file destination, the file could not be created. I have tried exporting as png or jpg, and to different folders but get the same error. After closing and restarting Affinity photo, the problem is resolved for a while. After the fourth or fifth image being cropped it happened again. So restarting is a fix, but it seems like a bug.