You have set your escape character to "/". So when you do
CONNECT SYSTEM/MANAGER@DATABASE_B, Oracle treats character '/' as an escape character, so it thinks you want to connect as a user with username SYSTEMMANAGER and asks for a password for that user.

So before isuing CONNECT command, you must disable escape character or set it to some other character, using either
SET ESCAPE OFF or SET ESCAPE # (where you can replace # with any suitable character).