Iris 1.3.1 has released, fixing a crash with multiple mods when loading a world as well as improving key bind reliability.
Note to resource pack developers: This crash happened due to Iris attempting to replicate an unintentional Optifine feature named "broken paths", which allows uppercase letters and other normally disallowed Unicode as paths. We have switched to partial support; Iris may not support this properly in the future, please don't rely on it.