This looks like the error screen from the Chrome browser, so I think your idea of checking connections is a good one. Can you get to the SDM from the HMI? If so, is the PC 2100 in RUN mode?
It sounds like you’ve run into this error before; does it happen often? The HMI/browser usually loads faster than the server on the 2100 so it may just be the case that you need to wait a minute and refresh.