Trying to migrate assets from 4.17 custom build to 4.20 but it doesn't work

Hello, I’m trying to re-use some assets from a game made in UE4.17 (source code modified) to a new project in UE4.20, but after migrating the files do not show up in the Content Browser (only empty folders show up).

Already tried to delete the CachedAssetRegistry.bin but the files still do not show up.

I thought there should be no problem migrating assets from an older version to a higher version of UE4? Does modifying the source code somehow invalidate backwards compatibility?

Any ideas for what I can do here?