This bug was fixed in the package xfdesktop4 - 4.11.4-1ubuntu1.
From the xfdesktop 4.11.4 changelog:
* Better migration from previous versions:
- Xfdesktop now does a better job of migrating any user settings
from 4.10 and before to the new xfconf properties.
- Fix an settings issue when plug names aren't available
This bug was fixed in the package xfdesktop4 - 4.11.4-1ubuntu1.
From the xfdesktop 4.11.4 changelog:
* Better migration from previous versions:
- Xfdesktop now does a better job of migrating any user settings
from 4.10 and before to the new xfconf properties.
- Fix an settings issue when plug names aren't available