The oVirt Team has just released a new version of ovirt-engine package that fixes upgrade related issues.[1][2].

We recommend to users experiencing upgrade issues to try again with this new release.Thanks,

[1] Bug 1609839 - Foreign key constraint violation on upgrade to 4.2.5
[2] Bug 1613168 - On rollback of failed upgrade from 4.2.1+, engine-setup outputs errors about the uuid-ossp extension
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