Web application is unable to return image file from temp directory

My web application creates avatars from the initials of the user and a random background color. On my local environment everything works fine, avatars are created and show up on the page.

On my test environment they do not show up however. The response is HTTP 500 but no errors show up in the log. I have added extra debugging messages but the only information i got from that is that the returned contentlength is 0.

I do see that the avatar is created again when I remove it from the temp-directory where it's stored so the application's rights are good it seems.

I can't get my head around what's causing this issue.