Can't insert any image via WYSIWYG

[TNTH]

Haven't been here for a long time due to all kind of circumstances (some good, some not so good), but it's good to see that the community is growing and the cms as well!

Yet I have a very strange problem. On a server of one of our clients it is unpossible to insert an image via a WYSIWYG-editor. I tried different versions of FCKeditorX (0.4, 0.9.7.b9 & 0.9.7.b10) and TinyMCE, and changed some attributes on files and folders. But that didn't solve my problem.
When I'll try to insert the image in my browser the pop-up opens (sometimes) but, no content appears.

We're working on an FreeBSD-4.10-server with PHP 4.4.0 hosted by Xs4all (nl). I can give you some other specs if needed.