Stage 6 of in place upgrade 4.1 -> 4.2 failed

Do you have some custom qrexec policies in dom0?
Maybe the upgrade script can’t convert some of your policies from old policy format to new policy format.

Comment from one of the linked github issues:

Summary of the problem: an ask rule with default_target= requires target=. If no target= is used, exception is thrown`, if it is used, only one qube can be selected.

An example of a problematic old policy rule: