I propably found a bug in FinalBuilder 4 with XPATH (an other ?), but I don't really know wether it is an FB bug or not.
The problem occured using an xpath iterator with msxml version 3 : with this version the iterator starts at 1 rather than 0. As a result when having 3 items to Iterate it starts from to 3 but as the index of the third item is 2 (0,1,2) the action failed.
To solve the problem I had to install msxml4. But I'd like to know wether it is an FB bug or not.