AmigaBruno, i'm sorry to hear that you have these problems, but since you already made a lot of successful transfers, i don't think that the problems you encounter can possibly caused by the cable or software.
After what you told us, i've update the ebay listing putting the warning note i already had (about having both pc and amiga turned off before connecting cable) to a much more visible place,and adding never to unplug cable when amiga is turned on, just to be "on safe side"!
Now about the holding screws,maybe you are right that i should put them too, but i assume there was no need to put these, as in most cases, someone will have the cable connected only when he make transfers, so screwing/unscrewing on both ends,every time he needs to make some transfers, (screwing only on amiga side,doesn't prevent the possibility of unplugging on pc side, so both connectors need to be screwed), is a rather tedious task that no one would do!
Moreover, the 25pin Dsub connector fits very tight on both amigas and pc parallel ports i've tested, so i never thought this would happen to anyone.
I really hope that parallel port is not damaged, although i believe that damage will possibly be made in the "opposite" case of yours, e.g. connecting the parallel cable while BOTH pc and amiga are turned on (although i personally, unplug/plug cable many times when only amiga is off and everything seems to works fine!).
p.s. Since you bought the kit with the pci parallel card, try to use it too, and see if it works. Drivers can be found
here . The specific pci card has been tested with my pc and works perfect!