1. Don't use Feather Client.
2. You should post the stack trace, a crash report, or the latest.log. The button to copy the stack trace is right there
3. ReplayMod is just not compatible with OptiFine/OptiFabric. There's countless issues already covering it.
4. You may want to consider switching to
   alternatives to OptiFine for Fabric

   
   https://lambdaurora.dev/optifine_alternatives/
   https://fabricmc.net/wiki/community:optifine_alternatives
   https://github.com/TheUsefulLists/UsefulMods
   https://fabulously-optimized.gitbook.io/modpack/readme/give-up-optifine
   https://github.com/skywardmc/additive/wiki/Give-up-OptiFine

1. Don't use Feather Client.
2. You should post the stack trace, a crash report, or the latest.log. The button to copy the stack trace is right there
3. ReplayMod is just not compatible with OptiFine/OptiFabric. There's countless issues already covering it. I recommend switching to alternatives to OptiFine for Fabric
   https://lambdaurora.dev/optifine_alternatives/
   https://fabricmc.net/wiki/community:optifine_alternatives
   https://github.com/TheUsefulLists/UsefulMods
   https://fabulously-optimized.gitbook.io/modpack/readme/give-up-optifine
   https://github.com/skywardmc/additive/wiki/Give-up-OptiFine

Okay, stop spreading misinformation for a start. Its not that ReplayMod isn't compatible, Mixins just need updated on Optifabrics side. Theres also a working build in #1112 someone posted, which removes the problematic code from BOTH replaymod and Optifabric, and fixes the incompatibility.

While I do agree that Optifine/Optifabric should be replaced, you also don't need to just spam post the same links in every single issue that gets made. Just cause your recommendation is to switch, doesn't mean someone wants to go through each update having to install 30+ mods just to match with Optifine, or constantly setting up new versions of the same client only to customise it anyway. Unless you actually have something constructive to add, just don't comment.

In regards to OP, see #1112 which has a fixed version of both mods you can use.

Thank you very much for educating me!

There's no difference to end users in why OptiFabric is incompatible with ReplayMod and the fact this is a duplicate issue still stands.

Thank you very much for educating me!

There's no difference to end users in why OptiFabric is incompatible with ReplayMod and the fact this is a duplicate issue still stands.

The more you know, no problem though.

agreed on the duplicate issue part however, as a search of the issues shows at least 30+ that are duplicates of this.