Happened to me too. If you complete the quest '[Bug Brawl] Bigger, Better Bugs' Aphids stop dropping, so if your Ladybug then loses level you can no longer transform it and it's impossible to progress in the event.
Possible fix would be to give everyone access to the quest '[Bug Brawl] Debugging', which resets your bug to Ant and album to Ant Photo Album. Or just make Aphids keep dropping.