Generated Course Files lockup (v8.0)

I've just got hold of the 5X, having has a 3 for several years, and on the whole am enjoying it, though still working out a few things.

One of the features I was interested in evaluating was seeing if the round-trip course generation was any better than on other Garmin devices I have used in the past. I have bought and downloaded the TalkyToaster routable maps and am using these for routing bike courses. I can get the Fenix to generate the routes and can 'go' if I want to navigate them but what I want to do is evaluate them on a PC for now. However if I click 'Save' then the watch fatally locks up and can only be recovered by a hard reset. After restarting I can see that a route has been saved but the watch thinks it's 0.0 miles long and if I try and load it then the watch locks up again.

I can mount the watch over USB and inspect the .fit course files but although they are ~15k in size nothing will successfully read them so it appears that they are corrupted during saving.

Has anyone else come across this kind of problem, or have any suggestions for things to try?

Thanks, Phil.