Good news...
after spending a whole night testing, my suspects were concentrated on the USR router. Its firmware was upgraded only a few months ago, but seeing that a more recent release was available, I've tried the new upgrade and it did work at once.
After rebooting, I got immediately the Limewire connection.
So, for all U.S.Robotics users with connection problems, my suggestion is to check for new firmware releases, before wasting a good sleeping night