Further DCDIAG shows the following errors:
The following domain controllers have not reached Global state ('Eliminated'): Domain Controller (Local Migration State) - DC Type =================================================== WIN2016DCNAME ('Start') - Writable DC Migration has not yet reached a consistent state on all domain controllers. State information might be stale due to Active Directory Domain Services latency.
And finally I find the following error in C:\Windows\debug\DfsrMig_x
Starting test: Services Invalid service startup type: NtFrs on WIN2016DCNAME, current value DISABLED, expected value AUTO_START NtFrs Service is stopped on [WIN2016DCNAME ] ......................... WIN2016DCNAME failed test Services Starting test: SystemLog ......................... WIN2016DCNAME passed test SystemLog Starting test: VerifyReferences Some objects relating to the DC WIN2016DCNAME have problems:  Problem: Missing Expected Value Base Object: CN=WIN2016DCNAME,OU=Domain Controllers,DC=XXX,DC=com Base Object Description: "DC Account Object" Value Object Attribute Name: frsComputerReferenceBL Value Object Description: "SYSVOL FRS Member Object" Recommended Action: See Knowledge Base Article: Q312862 ......................... WIN2016DCNAME failed test VerifyReferences
It seems that the new DC is still waiting for the Migration to DFSR, although it already uses DFSR and replicates the SYSVOL. As FRS is not supported on Win 2016 I can understand, that the new DC cannot complete the FRS to DFSR migration. But I cannot believe that this status should be normal for that situation as it should be common to add Win 2016 DCs to a Win 2008 R2 Domain. The quoted Knowledge Base Article Q312862 is for Win 2003 so it does not really apply.
20181009 10:04:13.971 5148 MIGM 738 [ERROR] main Error: + [Error:9512(0x2528) Process main.cpp:602 5148 C Migration have not yet reached to a consistent state on all Domain Controllers] + [Error:9512(0x2528) ProcessGetMigrationState main.cpp:485 5148 C [b]Migration have not yet reached to a consistent state on all Domain Controllers[/b]]
It did not change the situation.
<DbgInitLogTraceFile: 6860: 1884: S0: 08:28:28> :S: Full pathname for c:\windows\system32\ntfrs.exe <Migrate: 6860: 1376: S0: 08:28:28> Returning DFSR Migration Local state: 3. <FrsPrintEvent: 6860: 619: S0: 08:28:28> :E: Eventlog written for EVENT_FRS_STOPPED_ELIMINATED_STATE (13575) severity: Error at: Wed, Oct 10 2018 08:28:28