Oh, great. So because you weren't specific enough when I asked you to, you might not even have the problem you have been thinking you have and we might have wasted that last hour of troubleshooting.
Back to square one.
Start Minecraft, go into a replay and place a keyframe. When it gives you the error popup, click on the "Copy" button, then go to https://paste.replaymod.com/ and paste the error there. Once done, click the save button, copy the link and paste the link (not the error) in this thread.