Time Machine not backing up to network

I have been using a Samba share as a Time Machine source for a while now. It was a Raspberry with a Western Digital USB drive attached. For various reasons, I needed to rebuild the OS image on the Pi. Ever since, I can't get Time Machine to complete a backup. I can copy, rename, and delete files from both the Time Machine share and other shares without issue. I've played with permissions on the file system (even doing 777, just to test). I've also done a fair bit of googling to find issues.

My MacBook is running MacOS 26.

The `smb.conf` is pretty stock. The relevant section for `[Global]`:

[Global]
ea support = yes

vfs objects = catia fruit streams_xattr
fruit:metadata = stream
fruit:model = MacSamba
fruit:veto_appledouble = no
fruit:nfs_aces = no
fruit:wipe_intentionally_left_blank_rfork = yes
fruit:delete_empty_adfiles = yes
fruit:posix_rename = yes

min protocol = SMB3
fruit:zero_file_id = yes

for the Time Machine share:

[timemachine]
  comment = Time Machine Backup
  path = /mnt/lotsadisk/backups
  fruit:time machine = yes
  browseable = yes
  read only = no
  writeable = yes
  guest ok = no
  force user = smbuser
  create mask = 0777
  directory mask = 0777
  fruit:time machine max size = 4T

Running a backup, I can see it create a temporary file on the share, and copy data over. It will fail, with most errors pointing to a permission issue (log below). As I said, I've opened the permissions wide to test, and, through Finder on the Mac, create, manipulate, and delete files on the share. The Time Machine log is attached. Is there any obvious thing I'm missing?