StarArmy:1576851785 wrote
Is there a chance that the server was really busy (lot of requests to Apache or something) when the error occurred? Does it happen every time or just occasionally?
It happens every time!
I know that I changed some issues with permissions on my server in recent weeks, and I'm sure it precipitated this error. But whatever the fact is, the bottom line is that the directory is owned and has www-data as user, and yet still this error occurs.