Make sure you have moved the AuditLog mailbox (new in E2016) to new database before removing the default first database.
Check with this...
Move it using this...
Check with this...
Code:
Get-Mailbox -Database DefaultFirstDBName -AuditLog
Code:
Get-Mailbox -Database DefaultFistDBName -AuditLog | New-MoveRequest –TargetDatabase New-E2016-DBName