Symptom
After upgrading to Nexus Repository 2.8 or greater, you may notice a startup failure with the following message in the log files.
java.lang.IllegalStateException: Could not upgrade capabilities.xml to Kazuki as Kazuki already contains capabilities
Cause
Nexus Repository 2.8 automatically migrates a configuration file at sonatype-work/nexus/conf/capabilities.xml used in earlier versions to a new capabilities storage called Kazuki. As part of this migration it moves the original file to capabilities.xml.old.
If for some reason another process creates a new capabilities.xml file in the same directory (like the default sonatype-work/nexus/conf/capabilities.xml file that Nexus 2.7 creates at startup) then Nexus 2.8 would refuse to re-do the migration, to avoid corrupting the previously migrated data, and stop.
Solution
Backing up and removing this capabilities.xml file should then allow Nexus Repository 2.8 to start up. You can then check the capabilities panel to verify the migration.
If you want to repeat the capabilities migration, stop Nexus Repository, remove sonatype-work/nexus/db/capabilities/capabilities.h2.db, copy the old capabilities.xml file back into the configuration directory, and restart Nexus Repository.