This was fixed in stop precompiling FileIO.load by JonasIsensee · Pull Request #637 · JuliaIO/JLD2.jl · GitHub and convert UnPack into an extension by JonasIsensee · Pull Request #643 · JuliaIO/JLD2.jl · GitHub
So only available in JLD2 0.5 or above.
Feel free to try and backport fixes to JLD2 0.4, but you probably want to be on JLD2 0.6 to avoid other fragile loading code. Any help updating packages to JLD2 0.6 is greatly appreciated. 7. Compatibility · Pkg.jl