And it's only one of the three .json files (the big one, >2MB) that's making the problems. The two others work fine.
I guess all files are copied in one transaction in MiXplorer which gets rolled back, because the last of the files can't be copied.
But I can't copy this file with Solid Explorer too, so this seems to be no problem with MiXplorer.
Sorry for the inconveniences, I'll try to investigate further.