I am seeing more or less the exact same issue, but not for all apps. Manually migrating generates the ECONNREFUSED error in the log. Noting this additional detail:
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<hr><p>HTTP Error 503. The service is unavailable.</p>
All Qlik services are fully available.
The port (4232) is active per NETSTAT.
While updation i created a new shared folder in c drive for shared persistence storage. beacuse i dont know, previously where they have used the shared persistence folder.
You need to copy the apps from the old shared folder to the new shared folder. Make sure that the Service Dispatcher is running and then in QMC press the Migrate button. Hope that will solve your issue