Guillermo,
We just purchased XP-SWMM, and your problem might be that you have more nodes in your model than your program can handle. For example, if your model has 110 nodes, but your program has been compiled for a maximum of 100 nodes, this could be the source of your problem.
You might also try calling or e-mailing XP-SWMM technical support at
(your international dialing code) 1-813-886-7724
info@xpsoftware.com
Bill Walker
Post, Buckley, Schuh & Jernigan, Inc.