Could not open ../public_html/FILE for writing

In Virtualmin I created the sub1 subdomain of domain.tld
I uploaded the files for the Akeeba Backup restore.
All files are owned by domain.tld:domain.tld.
From my PC, via Firefox, I launch sub1.domain.tld/ks.php and immediately get the writing error.
Could not open ../public_html/FILE for writing

Why?

==== Update
I’m sorry.
Disk Quotas was automatically enabled and the folder already contained 1GB.
I still can’t extract the file, but I don’t think it’s due to this problem.

SYSTEM INFORMATION
OS type and version Debian 12
Webmin version 2.105
Virtualmin version 7.10.0
Related packages SUGGESTED