OK, it reproduces (at least it did once).  Here's a link to the saved game.  My fleet is the 3 corvettes in the Macrinus system.

I have also encountered this issue.  The fleet that was unable to attack WAS able to move somewhere else and attack a different stack of ships, but neither before nor after was it able to attack the first set of 'invincible' stacked ships.

This sounds very interestingly like the problem of being unable to colonize a planet, especially after what I was told was the problem. I wouldn't be surprised if both problems get fixed with the same fix. I also would not be surprised if both problems needed their own fix.

Cool,

I decided to load up the game and thought for a minute that combat was not enabled yet.  

For the whole 200 odd turns before my game crashed I had no aggression, could not attack colonies ens ens.  I just went and expended what I did it conquered the galaxy with influence.