Hi Mark
I dug deep into my archives and found Actionscript1 files from around 2004. Like yours made with Flash MX. I tried to open them with Animate 2019 (v19.0). And if worked! Of course I got this message:

But otherwise my old FLAs opened without a problem. So it seems that something else is wrong with your old AS1 files. It is not AS1 itself which causes the error: ('Failed to open document'). Sorry that I made you download Flash Pro CS6. And a shame that I can't help you any further.
Klaus
I tried all the above, but there must be something wrong with the code / file. In the end, I recoded the whole thing from stratch, which had the benefit of getting familiar with AS3.
Thanks everyone for the advice and suggestions.
m