File locked - can't be deleted; Nextcloud is quiet slow (approx 9MB/s, even for big files)

Update:
I found this one here: Dateien lassen sich nicht löschen and the post from Ventusfahrer helped!
I could delete it, yeah!

BUT a new problem occurs.
The Nextcloud is running in an Proxmox-VM. I gave a 160GB Container to the Nextcloud-VM and I gave 90GB to my standard-user. Only 84GB have been shown as available, that would have been still ok, but after deleting of this locked file I only got 76.4 GB of available space left?? where’s the rest gone? I can resize it to a lower value, but not to a higher one. What can I do? And how could I expand the maximum available space to the 160GB?