It was detected when I was testing the modpack in version 0.0.4 alpha.

• When the player's entity respawn after a death:

Stack-trace warn: "Missing living capability";
Light error-line: "at com.craftingdead.core.capability.living.Player.lambda$get$4(Player.java:271) ~[?:0.0.4-alpha] {re:classloading}";
Full error stack-trace: https://pastebin.com/S1MMA5Pv;
Full client log: https://pastebin.com/UxiTSF15;

I was using forge-31.2.45 to run it and Optifine for better rendering performance.
(This happened even without using Optifine)

PS: I don't know how to fix it by myself, i'm not familiarized with Forge > 1.15.x-1.16.x data-storage system.

Maybe this post can help: https://mcforge.readthedocs.io/en/1.15.x/datastorage/capabilities/#persisting-across-player-deaths

This is a known issue, it's going to be fixed as part of a rewrite of some of the mod's internals.