Does that happen when you click 'New Character' or when you're opening an existing MQN file? If it's when creating new character then there's something wrong in the build and we'll have to check on the underlying code. But if it's when opening a character file, then maybe you can send your character file to hello@ar14.works so we can check what's wrong with the file.
For your information, in the meanwhile you can install the app using itch.io's official desktop app. It's quite a bit of a hassle initially, but it has the benefits of enabling auto-update for Mannequin whenever the itch app is running. Mannequin itself has no DRM, so you can just launch it after installing without keeping the itch app running at all times.