samba - Feb 2020 - vfs fruit disk_free fails on tmsize overflow with macOS Time Machine

For your version of Samba, you do need to include in [global]
min protocol = SMB2 
I see you tried it, but you need to keep it.
You don?t need the ?guest ok? or ?read only? parameters in your TMBackup config.
Mount the TMBackup share in Finder using the smb://User at
Server/TimeMachineBackup and leave it mounted.
From TM preferences, select the mounted TMBackup share.  Each time Time Machine
runs, it should find the mounted share. The TM share on my NAS is always mounted
and TM runs on schedule for my Macs.
You may want to check your server logs to see if your USB was put to sleep or
disconnected.