I succeeded in solving the problem.
After having tried all kinds of things both on TC and PC (including disabling services etc) I installed the netxfer software on another and totally clean PC.
It worked at once.
I haven’t been able yet to figure out the difference between the two PC’s. Maybe it has something to do with different Java versions (updates). But now the process of reimaging is working.
I was under the impression that the the TFTP process had completed because it was talking about the NAND flash stuff. But this is misleading. The netxfer program is quite involved.
Thanks for braining along with me.