One of our very valued partners raised a support ticket with Microsoft support. The migration from Dynamics 365 Business Central v15 OnPrem to Dynamics 365 Business Central SaaS stopped due to this error:
In application event log file we saw the following message in customer environment:
A majority of companies and tables did migrate successfully but only a few errored out. The root cause was that the company name had empty spaces at the end. After correcting this, the migration was successful.
Hope it does help others.
Thanks.