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OK. I had Windows 7 running with qBittorrent v4.2.1, all torrents completed and no partial downloads.
Wiped drive due to installing Windows 10, saved following folders beforehand on Windows 7:

C.users.username.Appdata.local.qBittorrent
C.users.username.Appdata.roaming.qBittorrent

Install Windows 10 and qBittorrent v4.2.1
When I now open qBittorrent all the files are visible except they are in red and all say missing files.
Can anybody tell me what the issue is here?

Never mind, solved it on my own.

How many torrents do you have? You could redownload the .torrent files and just point them to the existing locations.

It means the original location is not there anymore, you should:
1) Quit QBT
2) rebuild it
3) Move files there
4) and Start QBT, it'll probably recheck automatically and you'll be all set up

Yesterday I had the same. Shutdown qBittorent. Try going to your folder manually, with the filemanager. I got a message that I was not the owner, it will ask you if you want to open the folder, just say yes. It will take a few minutes. After that qBittorrent can acces the folder aswell.