I somehow managed to lock myself out from viewing the CMSPortletAdminInstance. It doesn't show up under "Portlet Instances" in the admin portal, and the CMS admin page is empty, yet the portlet seems to still exist. When I create a new instance with the same name, I see an error stating that an instance with such a name exists already.
Any ideas on how to give back myself these permissions, without wiping the database?
I already found and fixed the issue by changing portlet-instances.xml in portlet-cms.war. I have played around heavily with permissions, and I must somehow have turned things upside down.