i tried to replicate the error, but i couldn´t do it exact the same
procedure, the Disk was all ready filled up to 70%, and was not empty
like when the error appeared.
I couldn´t move the files away due a space problem.
I tried with my Musicarchiv (ca. 750 GB) but the error didn´t appear.
As a workaround for my problem (how i filled up the Disk) i created a
borg backup local on a USB 1 TB Disk (Btrfs Filesystem) without
problems and used Rsync to copy that backup to the server (happens
befor i got your mail).
On the Server i got the same confusing freespace by then, df, and KDE
Plasma widgets show me 0 Byte left, but Rsync finished without error,
and the borg repositorie is working troubleless remote.
As soon as i run again into that error i will do the procedure you
described me, and send in the requested datas.