[Plugin: WP-DBManager] Subdirectory install fails

I have wp-dbmanager operating perfectly in a root install, which is to say WP lives in /public_html/

I created a second site in a subdirectory: /public_html/site2/ (it is not a subdomain, just lives in a subdirectory).

Everything looks right and when on the Backup screen, everything is green, everything exists, is writable, is enabled.

But executing the back up results in:
Database Failed To Backup On … Backup File Size Is 0KB.

However it still sends me an email with an appropriately named file of zero length.

Two error files are generated, one in wp-admin and one in the subsdirectory which is site2’s root. The error files are only generated by executing a backup, not other functions.

Error from site2 root dir

filesize() [<a href='function.filesize'>function.filesize</a>]: stat failed for /path/to/public_html/wp-content/backup-db/12345_-_dbname.sql.gz in /path/to/public_html/wp-content/plugins/wp-dbmanager/wp-dbmanager.php on line 111
fopen(/path/to/public_html/wp-content/backup-db/12345_-_dbname.sql.gz) [<a href='function.fopen'>function.fopen</a>]: failed to open stream: No such file or directory in /path/to/public_html/wp-content/plugins/wp-dbmanager/wp-dbmanager.php on line 113
filesize() [<a href='function.filesize'>function.filesize</a>]: stat failed for /path/to/public_html/wp-content/backup-db/12345_-_dbname.sql.gz in /path/to/public_html/wp-content/plugins/wp-dbmanager/wp-dbmanager.php on line 114
fread() expects parameter 1 to be resource, boolean given in /path/to/public_html/wp-content/plugins/wp-dbmanager/wp-dbmanager.php on line 114
fclose() expects parameter 1 to be resource, boolean given in /path/to/public_html/wp-content/plugins/wp-dbmanager/wp-dbmanager.php on line 115

Error from wp-admin:

filesize() [<a href='function.filesize'>function.filesize</a>]: stat failed for path/to/public_html/backup-db/12345_-_dbname.sql.gz in path/to/public_html/wp-content/plugins/wp-dbmanager/database-backup.php on line 70
unlink(/path/to/public_html/backup-db/12345_-_dbname.sql.gz) [<a href='function.unlink'>function.unlink</a>]: No such file or directory in path/to/public_html/wp-content/plugins/wp-dbmanager/database-backup.php on line 71

Note the paths in the second error set are only different because I tried moving the backup-db dir to see if it had an impact. Didn’t.

PHP was recently upgraded to most current stable, by my host as we were using an “end-of-life” version.

Could something have been done or happened in that process that would cause this behaviour?

It seems odd simply because everything works perfectly on the other site on the exact same system.

If anyone sees something obvious, I’d appreciate the tip. But no deep digging is required as I see the author announced on his web site that he is moving away from supporting his plugins so I’ll have to replace it anyway. But’d be nice to put that off…

https://www.ads-software.com/extend/plugins/wp-dbmanager/