Error on upgrade NAV2016 CUM3 to 365BC SPRING CUM4

I am trying to upgrade a NAV2016 *** 3 database to 365BC Spring *** 4 database.

I follow the MS description as found here,

Everything works fine until Task 11, Synchronization. It runs for a short while and then ends with the following error:

“Sync-NAVTenant : A dead connection cannot be brought back to the livings”

Database size is 85GB. It should not be a SQL command timeout because I increased it.

I have tried the process on four different machines and get the same result. Using SQL Server 2014.

The NAV2016 database works fine and operational. I can compile all objects and synchronize before I start the upgrade.(I have attached screenshot with here)

Do you have any ideas?

Have you changed the SQL Command Timout from 00:30:00 to 23:59:59 on the servicetier? (I have had the same error but changing it to 23:59:59 solved it for me)