Hi!
Did you start the migration from the WebUI?
I had the same issue importing around 56k tickets from OTRS.
My suggestion is
- to set up the nginx server timeouts accordingly
- tune zammad workers and concurrent jobs Configuration via Environment Variables — Zammad System Documentation documentation
- use the console as explained here From Zendesk — Zammad System Documentation documentation
I set up a staging installation first and tried it a few times until everything worked fine.
Have fun!