Firefox Portable Beta (26) to Aurora (27) update broken

Hi,

I had Firefox Portable with the Beta Update channel running. It was the Original portable apps FF Beta package. I had no problems using the FF update process including the now up to date version 26.

Now I have switched the update channel to Aurora (to test the new shumway flash alternative). But the update from FF 26 to 27 is not working.
I get a window saying 27 is available (or press check update button), the downloads starts an completes and then a "applying update" and "update failed" Message.
Restarting the update process starts the full download again and results in same messages without any other hint why...

OS is Win8

see you Xyne