I have no idea what is happened, it worked fine some time ago.
ADDED: I tried to upload an image with Manage>Uploads link but I don’t see the “Upload” tab there – I can only browse uploaded images.
Finally I managed with uploading a file by copying the IFRAME URL from page source (upload.php?style=inline&tab=upload&post_id=XXXX) and opening it in a new browser window.
The ‘unknown property’ CSS errors are normal; they’re caused by IE-specific hacks in the stylesheet code. It sounds like the page within the iframe was temporarily unavailable for some reason; are you still having trouble?
I’m not sure we’ve ever been able to upload from the Manage/Uploads tab, either. Which is a bug, but at least it’s a bug that afflicts everyone.
Best bet would be to send in an email to support at this domain with the specifics that you lists here as well as a pointer to this thread. That way you can be sure that staff is made aware of the issue.