Just an update, not sure it's relevant to the new Miranda version or new dbx driver version.
I had problems with it (see for example
https://forum.miranda-ng.org/index.php?topic=5434.msg21410#msg21410 ), because the .dat file was locked and couldn't be backed up with Miranda running.
However it is, since the last update, something has changed. Now, windows
can copy the file even with live Miranda, and the db_autobackup plugin started working too!
I had problems with this locked database issue from the moment I installed it after the database driver change (because I lost some data). Now, I managed to have one auto backup made since the update. I hope it keeps working.