urgent cross forest migration

Hi Experts,

i have an urgent issue with mailbox migrations from another forest to our forest.
Please can you help me out ?

All mailboxes now in Forest-A on a EXCH Server 2016.
The company was bought.

Now we have to migrate all mailboxes to Forest-B to EXCH2013 Server.
I have tried this with a cross forest migration with the script .\PrepareMoveRequest.ps1
But it is not working, see the error here:

MigrationPermanentException: Error: MapiExceptionCorruptData: Unable to write mailbox info. ‎(hr=0x8004011b, ec=-2147221221)‎ Diagnostic context: Lid: 1607 StoreEc: 0x8004011B Lid: 1494 ---- Remote Context Beg ---- Lid: 36024 StoreEc: 0x8004011B Lid: 59009 StoreEc: 0x8004011B Lid: 53912 StoreEc: 0x8004011B Lid: 1750 ---- Remote Context End ---- Lid: 9954 StoreEc: 0x8004011B 

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More Infos:
The Two-Way Trust is established.
The MRSProxies are enabled on both EXCH-Servers.
The Endpoint is created on the Target-EXCH-Server.
The Script ( PrepareMoveRequest ) works but the Migration with ECP GUI not.

Please help me out this is really urgent.