Project transfers on legacy storage
The project transfer does not work on legacy (not hashed) storage with Heptapod 0.12.0 RCs. This will be fixed in the 0.12.0 final release.
The hashed storage becomes the default in GitLab 12, that's why it went under the radar.
To upload designs, you'll need to enable LFS and have an admin enable hashed storage. More information