I've tried on several occasions to update IJC to 5.3.7, but without success. The download always gets stuck at 3%. I have an admittedly slow internet connection, but never had this problem before. I'm using the now-webstart version.
This is using autoupdate? If so I think it must be something due to the connection, as updates seem to be working fine in general. Is there anything strange about your nework setup that could affect this (proxy server, firewall...).
If you still have the problem you might try downloading the contents manually, unzipping and loading from those local files.