Like I said I am not even able to switch to Boost. I didn't change anything on the admin dashboard except CSS, where I should start to find the source of problem?
See, https://docs.moodle.org/dev/Moodle_4.0_developer_update#Theme_changes
See, https://docs.moodle.org/dev/Moodle_4.0_developer_update#Theme_changes
I don't see any admin dashboard related changes.