Hello Enas!
That's actually intended behavior, credentials are salted and hashed in the database, so taking DB A into client B without the complete data directories won't work. The only way is to fully copy the clients data directories as well, as they contain the required information that prevent a "stolen" database to be used.
Please read more here:
https://www.ilias.de/docu/goto.php?target=wiki_1357_Secure_Password_Storage
Cheers,
Max