Okay, so finally I was able to successfully debug the issue, this is what I did:
Keep project settings, basically ignore errors, since they do not point to the actual culprit.
- Run the editor Launch option with for instance a minimal project configuration, i.e. a single map migrated to a fresh project, including copy over config files, plugins.
- Remove from the minimal new project for debugging purpose, folders until the project is no longer able to Launch. If you still can Launch, re-add the previously removed project root folder. Launch is a sort of in-project compile/deploy option.
- Once the Launch starts to fail their is a high chance the current removed folder contains the project corruption.
- Now re-add the suspicious folder, and start removing sub-folders, until you can isolate the project threat.
In my case, this method yielded a root AI BP and a Enumaration BP. The actual error is related to a certain child AI BP in my project, and removing some blueprints resulted in successful Launch deployment, and because the issue was found, subsequent successful packaging. Because Launch works even with errors in the log, packaging requires to fix those first.
It is still unclear why exactly the AI child BP started corrupting after upgrading from 4.23 to 4.24 or .4.25, perhaps I can shine some more light on this later.