I don't know what changed. it used to work fine
Sorry for the inconvenience! I had defined all functions provided by CharacterModHelper in data.lua, so modders could see all relevant definitions in one place. However, some mods exploited that by calling function check_my_prototypes() during data or data-updates instead of waiting until data-final-fixes (as pointed out in the documentation). In version 2.0.6, I've made some changes that require the function to be called in data-final-fixes. This change has been documented in the changelog, along with a fix (basically same thing as your error message: call check_my_properties during data-final-fixes).