Problem lies in the WB_PATH and WB_URL, these are needed for when a website resides in a subdir, iewww.domain.nl/somedir/
You could manual change:
$Config['UserFilesPath'] = WB_URL .MEDIA_DIRECTORY ;
$Config['UserFilesPath'] = MEDIA_DIRECTORY ;
If a subdir is used change it to:
$Config['UserFilesPath'] = '/somedir' .MEDIA_DIRECTORY ;
I tried and it worked at my end. xampp on windows xp
Worked with pictures and file-links
If it works with you lot, maybe add it to the created ticket to get /somedir in a variable like WB_URL, maybe WB_PATH or WB_ROOT or something.
This can be used in FCK and Xinha and TinyMCE to make them userconfigurable
Maybe add it as a setting in the backend so changes could be on the fly.