Gist with logs and mods list (including versions and potentially related mods): https://gist.github.com/kd8lvt/e87fb120c0aaed8cf3b718392e0c5b7b

When rendering Entity Banners in Roughly Enough Items, my game crashes, with the above logs. I'm not sure if it's just the Aether being wonky or if it's a problem with REI. This happens with and without Iris Shaders,

Steps to Reproduce:
Install my mods list, and type @Banner in REI.

Potential Fix:
Add error handling and logging when rendering an entity banner would otherwise crash. This would also make it easier to blacklist misbehaving banners as a side-effect. Alternatively, don't render the entity when in REI.

The problem isn't related to REI. If you open the Entity Banners creative tab, or get the banner in survival it'll also crash.
https://gist.github.com/kd8lvt/e87fb120c0aaed8cf3b718392e0c5b7b#file-crash-2021-09-09_11-03-17-client-txt-L56
Here you can see the entity which caused the crash. Adding it to the blacklist in the mod's config file should solve your problem.

how does one add to the blacklist in the mods config file?

Having this same issue with the same mob, the "flying cow" from the Aether. I tried blacklisting it, but no dice. maybe I formatted it wrong, but I though it'd be better to ask how I could blacklist this specific mob.

Having the same issue. I have blacklisted every entity from the Aether and it still crashes. Here's the latest.log.