Good News! Managed to reproduce it perfectly. The key idea was: If Im convinced that movement is not interrupted by alien sight, why not leave aliens out of it completely? No frame perfect inputs involved either, can do the click as slowly as I want. I only do not know where the randomness comes from if Shift+Click alien rather than Shift+Ctrl+click is used. One prerequisite is probably that the soldier needs to be able to fire at the target before moving, which is not always the case, maybe there are some others I am not aware of.
These are the reproduction steps that make the game crash every time:
Start a new game, place initial base
fast forward until there appears a ground mission and start it - there will typically be no aliens in sight (if there are, you can start over, but it will still work most of the time)
select any soldier
prepare a movement command (just one step out of heli is enough) with one left click (screenshot 1)
hold Ctrl+Shift on a forward tile that is already in sight (Screenshot 2)
click left - soldier does the move (screenshot 3)
let go Ctrl+Shift
hold Ctrl
left click
Attached: output.zip
The other crash error message with grenades can be reproduced the same way, slightly modified steps:
Start a new game, place initial base
fast forward until there appears a ground mission and start it - there will typically be no aliens in sight (if there are, you can start over, but it will still work most of the time)
select any soldier
left click grenade slot
press and let go Ctrl, so you still have the grenade selected but are out of the forced throw mode
prepare a movement command (just one step out of heli is enough) with one left click
hold Ctrl+Shift on a forward tile that is already in sight
click left - soldier does the move
let go Ctrl+Shift
hold Ctrl
left Click
Attached: output_nade.zip
Screenshot 1:
Screenshot 2:
Screenshot 3: