I'm afraid I don't have an aswer to your question,, hopefully others in the community will. I would like to point out however that getting Atrium Discovery 7.5.01 working will not give you a head start when you decide to upgrade to Atrium Discovery 8.x. The two are very different products and skills in one do not translate to the other.
You're right Nikola and we are aware of all the things that are new/different in 8.x versions. Its just that we have this 7.5 setup and running and I was wondering if I could make it work. Weird its failing (probably) at WMI without giving any error.
It will be helpful if you can mention what configuration you have done for Discovery Type while configuring Discovery task. You should have a look at the logs of discovery to see what message does it give. Inventory logs and Discovery logs should give you messages on the details of discovery task.
So the issue came out to be that we enabled NMAP port scanning in discovery task config while our FW wasn't allowing this. We only requested WMI ports to be allowed over firewall while with NMAP scanning turned on, discovery opens a number of random ports to check service ports before connecting. Probably to not waste effort in unavailable ports.
Once I disabled NMAP scanning, it directly connected to WMI ports available on server and was able to scan everything.
We also enabled trace logs for agent and manager to find out where it was failing/doing nothing.