Kiwidude,
It looks like Manage Series is broken (running Calibre 0.9.20).
I can select a number of books to fix the series, it will let me shift them up & down in the list to correct the series index, but as soon as I try to lock an index, it bombs with the following error:
Update: it seems to happen when there is a book in the list that wasn't part of the series originally (i.e. it doesn't have a series & series index yet). If all books already have a series and series index things work fine.