Thank you for the Value issue. Actually U++ XML functions let to change Xmlize() functions without throwing errors. That is excellent because it permits developers to install new versions that will handle gracefully old user xml files.
This was in my case the reason of finding the error.
About the exceptions: XML files can be changed by hand by users. It is good to know not only that the file format is wrong, but what is the failure and where is it to guide the user to solve it.