Chunks have size of 0

I get an error saying that several chunks have size 0. At least one of those chunks is part of all revisions for that snapshot.

What’s the best way to fix this?

You should remove these chunks manually from the storage. If check reports missing chunks, follow this guide: Fix missing chunks

Which storage backend is this?

This was via SFTP. But I believe it may have been caused by usb resets/power issues on the external SSD.