Run fsck on your disk. I had the same problem, digging into it I found APFS had hopelessly corrupted my disk, its snapshots, and the time machine backups. In the process I not only lost mail and other files, but I had unexpected app crashes and casual kernel panics. Once I found the corrupted files I had to log in into single user mode to remove them as I couldn’t even as root when logged as usual. Don’t forget to delete your APFS snapshots, if you don’t the problem will quietly sneak in again. After that you’re safe to make a new snapshot and a new backup.
Why didn’t Apple just give us ZFS instead of the shitshow APFS is?
Why didn’t Apple just give us ZFS instead of the shitshow APFS is?