I was recording myself with the mod for a few hours of me building a taiga cottage. I disconnected out of the server to allow the mod to save the replay until an error erupted and couldn't save the recording ultimately losing all of that footage. It happened multiple times while I was idle in the server and wanted to disconnect. It seems that the bug can only be recreated for longer recordings? I recreated the bug so I could copy the crash report but it seems I personally cannot paste it here, but it is entirely the exact bug others are experiencing in which your developers had attention for the last several days.
Mods I'm using in the fabric-loader-0.4.8+build.155-1.14.3 launcher is listed below:
Optifabric-Mod-1.14.3 (to allow Optifine to be compatible with the 1.14.3 Replay Mod or Fabric)
The two threads you linked actually have slightly different symptoms, so it cannot be "entirely the exact bug others are experiencing".
Guessing from your description of the symptoms, it's probably the first thread. I've updated my reply there to mention the differences to the second thread.
The issue is not related (directly, anyway) to the length of the recording. Have you read my reply to the second thread you linked? Are you certain you actually have the same issue?