Hi @abbas,
thank you for your reply. I actually do not want to edit the core DocType, I only want to customize them, i.e. add custom fields. Clicking e.g. User > Customize however leads to the above screenshot and does not allow to edit. Also Custom Field List > New on DocType User does not work (same error dialog). Therefore I conclude that customisation of core DocTypes has been surpressed. This has been used (and I am happy that this is kept on updates and fixtures), but I fail to understand why this has been prevented…