As I already posted here -> https://area51.phpbb.com/phpBB/viewtopic.php?f=99&t=41639&p=251835#p251835 . There was/is a problem with attachments when you preview your post. The steps I did:

I quoted imkingdavids post and typed my reply. I added an attachment and placed it inline. I hit the preview button and noticed in the preview that the image was not shown but instead a "button" with the image name on it. Out of curiosity I clicked on it and it greeted me with the attached message and error lines in the header. I hit a coupe of times the back button to get back to the posting page and was able to post the reply nevertheless without having to re-upload the attachment. Unfortunately I was not able to reproduce this behavior, so maybe I was just lucky and the server had a hickup that moment?!