Freerouting crash on second run

I have had a repeatable problem with FreeRouting on a couple of my designs. In each case the first attempt works pretty well with a couple of fails. I load the session file and manually complete the board. Then a change comes along and I have some new nets to route, so I create a new dsn file and try to launch FreeRouting again. This time I get hundreds of warnings about polylines requiring two points and then FreeRouting stops with an exception.
Anyone else seeing this?