Thanks Paul!
I was looking at a session from the last Lotusphere, it's on the 22th slide that XPage error messages should pop up in the status bar (http://www.slideshare.net/ddrschiw/ad108).
It's so strange that there is no possibility to launch a debugger for client side javascript... Isn't there any add-on for the Notes client?
Thanks once more for your answer,
Adam