Hi Maurizio.
I have a test environment with both Windows and Linux with IBM MQ version V9.4.2.0,
The command is correct, following the documentation. In my case, the user is mqm, and the MQ manager is QMA.
My thoughts about your issue that you can try:
Remove both '' I'm not sure if it's acceptable in the login.
Do not use a root permission or an Administrator account to login, for instance, in this example [Magadmin@B80-C-PMAG01 ~]$ Execute strmqm RPASYSTEM and then runmqsc -u Magadmin RPASYSTEM
In a Windows environment, as an IBM MQ Server, create a service account to use IBM MQ and assign the user to the mqm Group.
Use a strong password with more 15 characters.
It is all I can think about that could impact the login.
Note: If none of those things work, please share with us, step by step, how you are doing the setup.
Best regards.
André Reis.