Shame on me, I forgot rule two: Google rules.

Double quoting the text reduce to 7 the number of results (my post here is the 4th!) and surely are related to my problem but give no solution (except the Hungarian one that I couldn't read).

But I'm relieved by the fact that it is not the SerialPort hating me and that in CB I've been given a workaround: explicitly closing the port before program end.