I made a simple water wheel (though it might affect all of Create) and everything was fine until I exited the world and reloaded back in. Once the world loads in, my world immediately exits to "Saving World" and the entire game crashes. The world simply cannot load anymore. Good thing it was a test world and there wasn't really anything important. I managed to track the issue to Optifine_1.16.4_HD_U_G6 since I removed it and I tried to replicate the crash and I was able to reload in a world where a Create contraption was present.

Here are two crash reports that generated when I was testing and replicating the crash:
crash-2021-01-22_19.39.10-client.txt
crash-2021-01-22_19.02.18-client.txt

This issue has been marked as stale because it has been inactive for 3 weeks. It will be closed if it remains inactive for another 3 weeks.

This issue has been closed since it has been inactive for 3 weeks since it was marked as stale.

I have the same problem, but with a windmill. I found out that this is really an Optifine and Create conflict. The error is caused by the moving part of the mill. If you save and exit the world in which the blocks move with the Create mod, then you will not be able to go back. My solution: turn off Optifine, enter the world, stop the movement of the mill, re-enter with Optifine enabled.

As a temporary solution, you can set removeErroringTileEntities = true in forge-server.toml (in the serverconfigs folder). I was having this issue, crashing each time I loaded the world, and as soon as I changed this I could load in.

The best thing is that it didn't even break any of the Create stuff that was there, it simply stopped it moving. It can be started up again as necessary.

Tried to replicate this crash by installing OptiFine 1.16.5_HD_U_G7_pre4 and I was able to load in worlds with Create stuff moving and working. Everything seems to be working, will update if I experience any more crashes. I don't know how stable this version of Optifine is because it is a pre release version so just a heads up if you wanna try it out.

Thanks for the heads up - my friend said they were all able to play on a server with Optifine and Create too so maybe this is what enabled that for them (though, it could also be the separation of Create on the server and Optifine on the client, rather than both on the client).