Excellent thanks chaps.
I've just pulled down the latest code base and all seems to be working again.
The URL of the forum is for example http://mypage.de/forum/. When uploading an image in a post, you'll get a broken image symbol instead of the uploaded picture.
The inserted path in the post is "uploads/files/1234567890123-picture.png". That's correct.
When checking the existence by entering the full and correct URL in the Browser it works. "http://mypage.de/forum/uploads/files/1234567890123-picture.png".
But when right-clicking on the missing image symbol, and selecting "open in a new tab", it tries to open this URL "http://mypage.de/uploads/files/1234567890123-picture.png". That's incomplete, because "forum/" is missing.