Also having the same issue with many modpacks, this error is not specific to any one modpack. It is a Twitch problem, and because of the massive amount of people experiencing and reporting this issue, I'm sure they're already aware of it and trying to figure out how to fix it.
I have had success circumventing this problem by backing up my save files and deleting the modpack profile, and then reinstalling the profile from scratch. This hasn't always worked, sometimes it takes one or two attempts. I just had another "Error: Cannot find child addon xxxxxx" error this morning, and used this method to fix it.