exchange databasecopies failed

What's the STATUS of this DB? Mounted?

Did you try reseeding with  -DeleteExistingFiles switch?
https://docs.microsoft.com/en-us/powershell/module/exchange/database-availability-groups/Update-MailboxDatabaseCopy?redirectedfrom=MSDN&view=exchange-ps

each server has one db and they are healthy.

But I cannot create a databasecopy.

-->But I cannot create a databasecopy.
What about your cluster? is that healthy.
Is it a stretched DAG? if yes how is the replication status?

Can you post the result of this command

Get-MailboxDatabaseCopyStatus *

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Get-MailboxDatabaseCopyStatus *

failed and suspended.

is it possible to create a DAG within different sites ?

Here is the error when I try to add a database.

The Microsoft Exchange Replication service encountered an error while inspecting the logs and database for DB1\DC1-VMEXDB01 on startup.
Error: File check failed : Database file 'd:\ExchangeData\Databases\DB1\DB1.edb' was not found.

[PS] C:\Windows\system32>Update-MailboxDatabaseCopy -Identity "DB1\DC1-VMEXDB01" -BeginSeed

Confirm
Are you sure you want to perform this action?
Starting asynchronous seed of database copy "DB1\DC1-VMEXDB01".
[Y] Yes  [A] Yes to All  [N] No  [L] No to All  [?] Help (default is "Y"): y

Confirm
The mailbox database copy 'DB1\DC1-VMEXDB01' has failed to update from server . Do you want to clean up that update request now? Seeding cannot
be requested for the same database copy until the failed request has been cleaned up by the server, which should automatically happen within 15
minutes.
[Y] Yes  [A] Yes to All  [N] No  [L] No to All  [?] Help (default is "Y"): n
The seeding operation failed. Error: An error occurred while running prerequisite checks. Error: A previous request to seed this database has
failed. [Database: DB1, Server: DC1-VMEXDB01.interwetten.loc] [Last error.: An error occurred while performing the seed operation. Error:
MapiExceptionNetworkError: GetPhysicalDatabaseInformation rpc failed. (hr=0x80040115, ec=-2147221227)
Diagnostic context:
    Lid: 55272
    Lid: 34792   StoreEc: 0x6BE
    Lid: 51176   StoreEc: 0x80040115
    Lid: 40680   StoreEc: 0x80040115
    Lid: 43980
    Lid: 36432   StoreEc: 0x80040115
    Lid: 52816
    Lid: 46672   StoreEc: 0x80040115
    Lid: 52264
    Lid: 46120   StoreEc: 0x80040115]
    + CategoryInfo          : InvalidOperation: (:) [Update-MailboxDatabaseCopy], SeederInstanceAlreadyFailedException
    + FullyQualifiedErrorId : [Server=VIE-VMEXDB01,RequestId=10d2ccf3-c3c0-45b5-aacf-848908dd08fa,TimeStamp=19.04.2020 21:27:39] [FailureCatego
   ry=Cmdlet-SeederInstanceAlreadyFailedException] 381A17E,Microsoft.Exchange.Management.SystemConfigurationTasks.UpdateDatabaseCopy
    + PSComputerName        : vie-vmexdb01.interwetten.loc

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Do you have any ideas ?

Please let me know what you did which will help others in future.
And apologize I missed to follow up this thread.

I want to setup a DAG over 3 different AD Sites.

any news ?