Matrix Authorization Strategy 3.0: "No type prefix:...."

I didn’t realize, that there is another plugin involved here: role-strategy.
And that does not have a compatible update yet.
(see [JENKINS-67387] "No type prefix: " in "Assign roles:" after updating "Matrix Authorization Strategy" to 3.0 - Jenkins Jira)
Sorry for the confusion - had been too late in the evening yesterday.
(I only had the chance to update plugins in the evening, when our users are out - so I tried to do that, just to run into this issue)

I got myself the admin rights back, by first stopping Jenkins the brutal way, by killing jenkins.war with no job running and then manually downdating the matrix-authorization plugin and hand-editing config.xml. (uffff…)