Your off topic answer
I created a shell script for a dedicated backup USB drive which runs rsync
on my 2 data drives. It's interactive, so I dry run first and evaluate everything that's changed. Before that, I run a different script to clean up junk (usually cache folders and certain log files).
For snapshotting the OS and applications, I literally tar up the system. That does mean in event of a disk failure, I'd have to manually re-create partitions and re-install the bootloader, but I'm comfortable with that.
Previously I used Clonezilla for snapshots, but I found it clumbersome for extracting a few files.
I can confirm that it's not launching for me either in 20.04. It does launch if I place the command into a shell script (.sh) and add that to Start-up Applications.
The removal of this line also works.
Version=1.1
So yes, a bug in MATE. Very good findings, thank you.
I'll create an issue on mate-session-manager
a bit later ... (unless I'm beaten to it)
If it isn't released in time, a possible alternate is for someone at Canonical to patch the deja-dup
package.