The PR #2129 changed the format of the snapshots. For the client-server communication this does not matter, since the protocol only uses snapshot deltas.
The demos however, use complete snapshots for the keyframes, so the current master cannot play demos from 0.7.3, same for the other way round.
Solutions that come to my mind:
1) Revert the PR
2) Convert the snap format when loading and saving demos
3) New demo format version