Mounting an exchange 2013 base from a new CU to an old one after an unsuccessful upgrade - is it somehow possible?

Question

System administration

Akhtung, help is needed very much.
There was an unsuccessful upgrade of Exchange 2013 to CU8, an error got out at the 10th step and the server died safely. After not the easiest restoration of the server itself, a problem with the databases got out of the backup: the databases managed to change the scheme to CU8 and refuse to be mounted. And it turns out that there is no backup for one of the databases.
Total: there is a working server with successfully restored databases except for one. The database was correctly shut down during the update, the database had a Clean Shutdown status from the very beginning, the Log Required / Log Committed parameters were set to zero, everything is beautiful and wonderful, eseutil /MH does not find anything wrong, eseutil /p runs successfully and does not change anything. Base logs and .chk deleted. The MSExchangeIS service restarted as needed. Creating a recovery database does not help, simply because everything is in order with the database from the server's point of view, the problem pops up only when you try to mount the database (accordingly, the recovery database is successfully created but not mounted).
Database connection error:
Failed to mount BlahBlah database. Error: Active Manager operation failed. Error: Database action failed. Error: Operation failed with message: MapiExceptionCallFailed: Unable to mount database. (hr=0x80004005, ec=1105) Diagnostic context: Lid: 65256 Lid: 10722 StoreEc: 0x451 Lid: 1494
Save-help, in short. So far, the only option I see is with a second attempt to update, but this is a very, very extreme option, since it lasts several hours and most likely will collapse again. There is no backup server, that's how bad it is.
PS details: the upgrade was from CU6 to CU8. After an unsuccessful update and rollback (the server is now alive, raised from a backup created before the restore), when using the get-exchangeserver command, it issues the CU8 build version.

Answer 1

The database format does not change when the CU is updated. The problem is probably with the file itself.
Delete all log files, all index folders inside the database folder (better move) - and mount.
Everything should mount if the base is in the "clean shutdown" state