For the record, I ran into this today, using UE 5.1. I have a blueprint deriving from my own Character subclass in C++.
- Opening the .uproject and trying to edit the blueprint: “invalid class” error message.
- Opening the .sln in VS and running the editor from there: no problem.
Not sure if this is related to Live Coding, but I had a ton of issues with reset properties, and even disappeared custom C++ components the day before when making substantial code changes using Live Coding.