In 1.21.6 I have been trying to use a fullbright texture pack. When I clicked done, it would say “Resources reload failed,” and I found this happens for every other texture pack. I booted up 1.21.5, and sure enough, texture packs reload fine.
Environment
Windows 11 Home 64-bit
Java 21 (Eclipse Temurin JDK 21.0.x Hotspot)
Minecraft 1.21.6
Prism Launcher and Vanilla Launcher (same issue on both)
GTX 1060 6GB GPU
Intel Core i7-8700H CPU
16 GB DDR5 RAM
No mods or OptiFine
Resource Pack: FullBright
It's possible that the resource pack you're trying to load is incompatible with the new version. Could you please attach the pack in question?