If you have multiple customers, who signed slightly different contracts at different times, and who also have different side agreements regarding specific facilities they may have paid for in part or in whole, you really need to define how you make allocation decisions in case of shortfall so that everyone understands their risk exposure.
I will personally hold the EC responsible because although AZ has also messed up by not negotiating a clearer contract, it is the EC that indirectly works for me and it is their job to think about this on my and my family's behalf. Had they understood the consequences of being so far back in the production queue much earlier they could have spent more on upgrading production but it is not clear to me that they even understood this until January. While I'm sure that they are trying their best, and bad luck with Sanofi plays a part here as well (as does the incompetence of e.g. the Dutch government in managing distribution), I am pretty unimpressed with how they have dropped the ball here.
It's not good them saying, "oh AZ isn't producing as much as they thought they would". Ok, cool story bro but they're making a novel biological, so you should have anticipated this and done more to prepare plans B, C, all the way through F. Not just placed a few commercial orders and then hope it all works out.