Thanks a lot for your answer. I know html syntax quite a well, but I suppose some files of mantis have to be modified to add target="_blank" parameter. Unfortunatelly I am unable to find where the change must be done.
sorry for my bad English, but I wanted to answer this question that cost me a lot to resolve and although I know it's old the publication has not been closed.
In order to help more collaborators in the future, I commented on the solution.