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If that does not work, please delete the files ij-ui-swing-updater-2.0.0-SNAPSHOT.jar, ij-updater-core-2.0.0-SNAPSHOT.jar and ij-core-2.0.0-SNAPSHOT.jar from the Fiji.app/jars/ directory (MacOSX users,
on the Fiji icon and select Open Package Contents and open the jars/ directory you see there). After that, the Updater should magically repair itself.
If that still fails, please download a fresh copy from here: