Hi Michal,
The server is definitely still on-line and I am able to see the expected green "Server online" indicator after choosing a model in my own instance of 23.1.0.455.

Can you give a bit more detail about the network environment you are trying this in? Is it likely controlled by a relatively security-conscious IT department? Is it possible to try again with the same computer in a less secure environment, e.g. if it is a laptop you can bring home? Is anyone else around you having success? Are you able to try it on other computers at work and at home? Do you see a difference with other computers and other network environments?

Despite your success with PowerShell and port 8500, if you have an IT department, you might want to check that they expect you to be able to connect to the proteomicsdb.org server on port 8500. This is still the most likely source of your problem.

Thanks for posting so much detail and your effort to find an appropriate solution on this support board.

--Brendan