on 1.21.3 on replay mod version 2.6.20, it records, but refuses to open the recording. It freezes at the menu, gives a network protocol error, or for server loops a loading terrain screen
Only replay and fabric 0.16.9 used.

Cannot reproduce. Please provide more information. In particular please attach the latest.log after trying to open the recording, and the recording itself.

Update:
it seems that worlds in superflat don't seem to work, while the default terrain works fine

latest.zip
the recordings, working and non:
replay_recordings.zip

I have an issue simllar to the description:

Here is my log:

https://paste.ee/p/ffz7O#s=0

Me too all sorts of bugs please look into it