"Unkown Cook Failure" despite 0 warnings and errors

I’m trying to package this project that’s like a menu “hub” for a couple of other VR Experiences. Basically I’ve built other projects in different product versions in the past. Then to be able to put them all into the same project I’ve updated them all to a later version and migrated them over to the menu project.

Most recently I put in the latest new project that was built in 4.21, and so the whole menu hub was also updated (including all the other projects inside).

As you might be able to guess updating and merging like this leads to some issues, however I sorted all those out and everything works fine inside the editor.

When packaging I also predictably ran into some problems, which were mainly that a few hierarchical meshes and enums had to be updated/replaced.
I did that an now have 0 Warnings and 0 Errors when packaging, yet somehow I still get

UATHelper: Packaging (Windows (64-bit)): BUILD FAILED

PackagingResults: Error: Unknown Cook Failure

at the end.

I’ve included the log as a txt-file in this post. Does anyone know what the solution to this might be?