My lock-freeze problem seemed to happen at random whenever I got past the "READY TO FIGHT!" screen, so I had a feeling that it wasn't the same cause as yours due to the very specific scenario that you've described. I assume you are using the Hackless method, which is what I used. I'm not too certain as to what role the SD card plays in the Homebrew version, so what I advise might not hold up if you're using that method instead.
I'm assuming it doesn't matter who knocks Falco out of his Side-B, just that the freeze initiates when the move is interrupted. Let me know if this isn't the case.
First though, check to see if Fox's Side-B produces the same effect. They're identical for all intents and purposes, so if it's just Falco, it is probably an issue with Falco's moveset data and within your SD card after all. If they both end up crashing, it might actually be a problem with the disc. Boot up regular Brawl and test to see if you can replicate it there too with either character. Test Wolf's blink-strike thing too to see if it's specifically an issue with all sorts of 'rush' attacks.
If it's happening in your normal Brawl game, then take a look at the disc and examine it for scratches or anything else that would otherwise obstruct the readability of the disc. This could be a scratch so tiny that it could be borderline invisible judging by how specific the crash is. If you have a friend with a Brawl disc, ask to borrow it to see if it works there as opposed to your original disc to rule out any possibility of a blemish completely invisible to the naked eye.
If all of this still fails to identify the problem, it really just might be your old beat up card. This is what I suspect to be the case, so prepare to find yourself a new one soon.
