Update on straightened and exported images
Yesterday Apple released OS X 10.11.5 and I wanted to know how well Aperture fares with that. It seems nothing has changed for the better. But the recently discovered problem that exported images which have been handled with the straighten tool (have a look here straight) has become even weirder.
But - I found a workaround. If you klick on the Auto Enhance button before or after you use the straighten tool, everything looks fine - at least on my system. After you clicked the Auto Enhance button you can do everything with the image - exported images won´t show any errors too. As Auto Enhance uses Highlights and Shadows a lot you have to deselect if it alters your image too much.
Apple does know about the straighten bug. I filed a bug report two weeks ago and received the answer that a record of that bug already exists. My file has been closed then. I don´t know, if that problem will be solved before 10.12 will be released.