resolved
Step 1 : create a new series in the Manage Series page (no icon, no desc)
Step 2.1 : Create a new post (fill title and text only)
Step 2.2 : Affect the new post to the new series (no part specified - leave the Series Part field empty)
Step 2.3 : Publish
[Repeat the Step 2 to get several article in the series - did that 3 times]
Until there, everything's fine : got 3 posts correctly numeroted from 1 to 3 (Check the Manage Post page).
Step 3 : Modify the last post (the third one in my case) to force its rank to 1 in the series.
Again, everything's fine, the 3 becomes the first, the first becomes the second, the second becomes the third and so on...
Step 4 : Edit the first part of the series (the last one before our mofication in Step 3) and change its category.
BUG !!!
Result : The second part of the series becomes (again the first one). So you have 2 part one in your series.
... Tricky no ?
