Samboli, That is the kindest and most elegant gift you could possibly give me - half a minute to insert the line of code and hours of frustration are over. Thank you!
I have not been able to use the built-in auto upgrade feature on this site because it has been throwing this error for each of the last few upgrades, but WPAU had continued to function until 2.9.2. One reason I have continued using WPAU in spite of the built-in feature on other sites as well is the ease of downloading the backup of files and db in WPAU. The auto download not only suits my lazy side, but it keeps me from fumbling around in parts of the site where I am absolutely out of my depth and skittish about making mistakes. If there is a security/safety reason for doing the db and files backups manually, though, as the WP built-in upgrade seems to require, I would be happy to know about that and change my ways.
Again, thank you for this perfect piece of advice and help.