Does the exported project work OK or is there a file missing? If so which file is missing and what was its image format in the image editor? Did you enable PNG recompression? These are the things that could relate to this bug.
@Ashley - I exported the project without any problems, then changed a value in a global variable, and then exported the project again. This time the bug occured. After that I delete the export directory and started the export again - this time successfull