It's best to choose generic non-branded custom pack (you know, without vodafone, o2 and other companies). Here is page where I wrote about my problem XS++ 2.x Guide - Page 10 - Top Sony Forums
I flashed my phone with xs and when it was stuck on file system activating i tried with setool. I was very confused, couldn't find any tutorial about setool so I clicked ... at the Firware upgrade pack: and browsed for main firmware, fs was also in that folder, then selected Flash and when it was successfull
I hope that will work for you same as for me