how to fix failed "emigration from artifactory to access"?

upgrading artifactory from 5.5.2 to 5.6.0 fails during "emigration from artifactory to access" with:

2017-11-17 11:27:38,614 [art-init] [INFO ] (o.a.s.a.e.AccessSecurityEmigratorImpl:42) - Starting emigration from artifactory to access
    2017-11-17 11:27:39,485 [art-init] [ERROR] (o.a.c.ConvertersManagerImpl:216) - Conversion failed. You should analyze the error and retry launching Artifactory. Error is: HTTP response
    status 403:{
      "errors" : [ {
        "code" : "FORBIDDEN",
        "message" : "Logged in 'jfac@01btna61r80s0n0y2ydfda1tyy/users/admin' is not an admin."
      } ]
    }

After restart, all user permissions and especially admin privileges are gone.

How do I restore previous user permissions or downgrade to previous version?

Solution

Worked around it by re-installing the previous version 5.5.2 and restoring all data and settings from backup.