Interesting one, mettulus, that sounds plausible.
It's interesting that you can remove the module from the original track, and then the clone with no issue. It's only if you remove clone first, and then the original that causes the crash.
I've been rushed off my feet for the last few days, but will try out some of the ideas posted here. I know it has been submitted to devs, and that Dan G is looking into it, so we should get an answer somewhere along the line.
Thank again, everyone.