In a regular folder the file previews load just fine, but whenever a file is in a plasma vault no preview is displayed.
If I move the same file outside the vault, the preview appears.
To some extend is behaviour is probably intended as it wouldn't be good to store thumbnails from files in an encrypted folder in ~/.cache/thumbnails,
but not having the functionality at all is unfortunate.

Is there any way to make this work without too much trouble and having the thumbnails stored inside the vault?

Thanks for the help.

Funny thing, I can't get it to NOT display previews in vaults. I absolutely do need dolphin to NEVER create thumbnails from files in vaults, and I can't achieve that.