Hello Juan,
“My understanding is IoT Gateway englobes OPC UA Server. So, the info above about OPC UA Server could be applied to IoT Gateway. Could you please confirm that?”
I’d rather not confirm nor deny this assumption, as I don’t see how it’d help.
Instead, I would like to recommend (as I did in my previous post) to read the manuals for OPC “Classic” Server, the OPC UA Server, and the IoT Gateway products respectively, and look for information regarding supported RobotWare.
For example, I saw in the OP UA Server manual (available here: Link) that it requires RW6:

And in the IoT Gateway manual (available in the same thread as the previous manual) this can be read:
The same section in the OPC Server manual (also available via thread mentioned previously) does not state any specific RobotWare requirements, thus it most likely supports RW5.x and up.
Furthermore, the OPC Server “Classic” was/is based on OPC Classic specification (OPC Data Access - Wikipedia), while the OPC UA & IoT Gateway are based on OPC UA protocol (OPC Unified Architecture - Wikipedia).
So the statement “The new OPC UA is really the old one with some new features and compatibility for the RW7 and above […]” is unfortunate.
Finally, I’d like to urge you, once more, to first and foremost refer to the existing application manuals rather than chasing down peoples’ opinions/thoughts. ABB provides application manuals for all their software products as far as I know.
Oh, I almost forgot your last question: “Also, I was wondering if a different than the controller IoT Gateway User ID info (User name and Password) could cause that issue.”
First, make sure you’re using the OPC Server software that supports RW5.15.