If your network connection is working fine, then you may have a partial download of that extension in your tce directory. In that case, delete the partially downloaded extension as well as it's md5sum file and all should be well. As you probably know, partially downloaded files are named the same as they are when fully downloaded.
Or, much less likely, somehow the permissions of you tce folder have been changed. But the first scenario is by far the most likely. There may be a check for tce folder permissions, but I can't recall and am unable to look or test for that right now.