Anyone know what the reasons can be for getting a 'Mission Not Valid' error message when everything seems to be AOK?

if you do no tick the box for transport,deploy,acs defend, recycle etc....not sure what else, maybe not enough deut to launch. Or if you try and send ur recyclers to a planet/moon instead of a DF.

all i can think of atm

Aha, you got it, click the box

And another rookie error, setting max to cystals doesn't mean that is what you want to collect, it just fills the cargo bays and means you haven't room to collect anything. Indeed your whole stockpile disappears if the ship is lost: Another non-intuitive monent! (imaging attacking with full cargo hold: the attack option ought to disable to the cargo option!)

Live and learn eh!

At least I now have a ship (gremlins gobbled all of them till now, as soon as building them finished! Still don't know why).