I had the same prob and I finally executed a part of docilweb's solution. Just downloaded the _v401.zip, unpacked it and uploaded the whole admin folder (deleted old folder before). That's it!
Opens again with dthe default page and as I used a custom template I just have to aplly the formating to the new one.
I backed up the files before, server is suEXEC, so no permission probs at all (in my case of curse). As I used a custom template (to adapt the client area to the outlook of my website design) I kept that unchanged but there may be a different config of the internal linking between the old and the new version of template. (To be verified!)
I will update my post when I find out more.