There's been no change at our end, and any Heartbleed-bugfix update by itself should not cause an issue, though you may want to contact your host to make sure they didn't take other measures that may have affected your remote keygen scripts. Maybe it's as simple as re-enabling execute permissions on your keygen script files?
If it's only your latest sale that failed to issue a key, that item's keygen URL isn't even using https://, just http://, so I don't think anything pertaining to Heartbleed would be relevant there; troubleshooting just that item's keygen script would be a place to start, rather than presuming all your keygen scripts have a problem if only that one has actually misbehaved so far.
If your keygen script failed to respond or threw an error, you should have received an email notification about that including the full error message output; however, you would not be notified if your script responded normally but simply failed to return any output.
