sometimes I think we tend to latch on to a specific detail and think it must be the most important thing, in this case the fact that the files are on a DFS share, it may be important but maybe not related at all
This thread seems very familiar, feel like I've participated in this conversation before and am fairly sure there are multiple cases of similar problems in these forums. From a quich search I found this thread which may help, but I'm sure it isn't the only one.
especially make sure the site is configured for anonymous and windows authentication. something different after login makes me think that is not configured correctly