Your comments

Very strange.

Can you, please, post IBM Explorer Connection Details -> Properties tab screen shot?

Anything in logs? On either side?

It should recognize -a in lsb_release. Ask them to run cat /etc/redhat-release then.

Please try without user ID too. Both user ID and password should be empty.

Which Linux brand/distribution is that?

You can try with  lsb_release --all .

OK, it is Linux, not UNIX, similar but not the same.

Can you ask for output of command lsb_release -a ?

Sebastian, I ment the UNIX server data on which MQ is running, not the MQ server data.

Please ask for UNIX OS specs. I can maybe reproduce it in my environment.

BTW, can you get more info on MQM UNIX server - UNIX brand, version, ...?