I tried to load up Quilt for 1.19.3 on the latest beta and I got a popup saying there’s an internal error and to report the error here. Sorry if the info I’ve provided isn’t correct, first time reporting a bug for something related to Minecraft.
Here’s a pastebin, the log is too long to post here
The problem is likely Frame API being incompatible with 1.19.3. Try removing that or downgrade to 1.19.2.
Since it’s an API though, removing it will likely cause some other mods to not work, but loader should tell you which mods those are.