Upgrade from 7.7 to 7.8 breaks admin perspective

After upgrading from 7.7 to 7.8.3 (with CMS version 2.24.03) the 'admin' perspective behaves oddly. When it is selected, a blank screen is shown, a ConcurrentModificationException is logged, and a second admin perspective icon appears. That second icon opens a working admin perspective.

Steps to reproduce:
1. upgrade a 7.7 project to 7.8 (e.g. the archetype)
2. start the upgraded project with the old (7.7.) repository already bootstrapped
3. login to the CMS as admin
4. click the 'admin' perspective

Expected: the admin perspective is shown
Actual: the selected admin perspective is blank and a ConcurrentModificationException is logged. A second admin perspective icon appears that opens a working admin perspective.

An easy workaround is to remove the node /hippo:configuration/hippo:frontend/cms/cms-admin/root.

This issue is caused by CMS7-7091 ("Backport: load perspectives lazily"). The cms admin perspective plugin moved from /hippo:configuration/hippo:frontend/cms/cms-admin/root to the /hippo:configuration/hippo:frontend/cms/cms-static/adminLoader, but the original 'root' node is not removed from existing configuration because the cms-admin cluster configuration is not reloaded. The workaround is to remove this 'root' node manually (as described above).