We could indeed improve the error reporting on this case, and even hint at the solution in the error message, e.g.: "If existing fiscal periods are already defined on the same calendar months (e.g. due to demo data), please delete them before using this wizard".
For the rest the current behavior is ok.
Keeping as Wishlist - priority to be re-assessed by Addons3 R&D team.
We could indeed improve the error reporting on this case, and even hint at the solution in the error message, e.g.: "If existing fiscal periods are already defined on the same calendar months (e.g. due to demo data), please delete them before using this wizard".
For the rest the current behavior is ok.
Keeping as Wishlist - priority to be re-assessed by Addons3 R&D team.