That bug refers to Ubuntu's refusal to install rootstrap (for reasons that aren't entirely clear from the refusal).
Either putting rootstrap back in the repository or removing the rootstrap dependency from pbuilder-uml would resolve this bug. From the bug above, the latter appears to have a greater chance of success.
This appears to be related to https:/ /bugs.launchpad .net/ubuntu/ +source/ rootstrap/ +bug/130640
That bug refers to Ubuntu's refusal to install rootstrap (for reasons that aren't entirely clear from the refusal).
Either putting rootstrap back in the repository or removing the rootstrap dependency from pbuilder-uml would resolve this bug. From the bug above, the latter appears to have a greater chance of success.