I use Neoforge 21.1.242 for MC 1.21.1.
I installed v1.8.4+1.7.3+1.21.1 and Navas ZA Megas 1.7.3 about 24 hours ago for testing out updating a modpack, not knowing that v1.9.0 would drop within 12 hours. At some point, despite working fine before, I no longer could open my testing world (or any world) because the game claimed the datapacks in the modlist were invalid, even after removing all of them, and if I tried to make a new world the game would crash. I also tried to do what the game recommended and tried to launch my world in Safe Mode, but this resulted in an error where the world data was labeled as corrupted, and the game refused to load it.
I eventually fxed the problem when I downgraded the mods to the versions I used the day before, but now I can't replicate the issue even after re-updating to the latest versions of the mods again and giving the new Mega Stones to my Pokémon and relogging the world, or restarting the game entirely. It may be possible that this problem only happens after giving a Pokémon a Mega Stone from ZAmega that was added to Mega Showdown with the new update, have the mods convert the item from the ZAmega version to the Mega Showdown version, and then restarting the game. But I haven't confirmed that.
Even with incomplete data, I though it worth to bring up just in case.
I use Neoforge 21.1.242 for MC 1.21.1.
I installed v1.8.4+1.7.3+1.21.1 and Navas ZA Megas 1.7.3 about 24 hours ago for testing out updating a modpack, not knowing that v1.9.0 would drop within 12 hours. At some point, despite working fine before, I no longer could open my testing world (or any world) because the game claimed the datapacks in the modlist were invalid, even after removing all of them, and if I tried to make a new world the game would crash. I also tried to do what the game recommended and tried to launch my world in Safe Mode, but this resulted in an error where the world data was labeled as corrupted, and the game refused to load it.
I eventually fxed the problem when I downgraded the mods to the versions I used the day before, but now I can't replicate the issue even after re-updating to the latest versions of the mods again and giving the new Mega Stones to my Pokémon and relogging the world, or restarting the game entirely. It may be possible that this problem only happens after giving a Pokémon a Mega Stone from ZAmega that was added to Mega Showdown with the new update, have the mods convert the item from the ZAmega version to the Mega Showdown version, and then restarting the game. But I haven't confirmed that.
Even with incomplete data, I though it worth to bring up just in case.