info.magnolia.ui.dialog.setup.DialogMigrationTask class was changed in 5.5 to reflect the refactoring of dialog actions (from package info.magnolia.ui.admincentral.dialog.action to info.magnolia.ui.dialog.action). So if we go by update we end up having cancel and save dialog action classes from the old package, but if we do a fresh install we end up with action classes from the new package.

Proposed solution

Configure dialogs using bootstrap files instead of DialogMigrationTask.