Comment 16 for bug 1727653

@Kenneth:

I increased ulimit to 4096 -> same error

Then I wanted to understand how ulimit works:

ulimit influences the shell (bash!).

I can start duplicity via deja-dup from the console -> works
I can start deja-dup from the launcher (GNOME-Shell) and manually trigger backup (from the running application) -> works

If deja-dup is started via automatic mechanism (daily launch) -> error

Concluding:

The issue does not lie within duplicity (well this generates the error) but in the calling mechanism of deja-dup (which must provide duplicity with different environments depending on how it calls duplicity).

If my conclusion is correct:

Changing ulimit will not help locate the problem
Getting debug traces via console commands will not help locate the problem
We are looking in the wrong place

Maybe the issue should be forwarded to the deja-dup developers to see how they call duplicity on auto-backup.