Sefaira plugin not running install - 500 internal server error

I tried installing Sefaira 2020 and followed the instructions from sefaira support ([]), I run through all the steps right up to where I select Extensions>Sefaira Plugin>Show Sefaira Plugin but the installer doesn’t open and all I get is this error message:

Assuming this is a backend issue - any advice?

Did you restart the computer, by any chance? Sometimes, this helps.
Do you have a VPN installed or firewall blocker?
Maybe it is a port that is not open.

500 indicates that a connection to the server is made, but that there was a crash in the server’s running code.

Thanks for replying both. Mike, I ran through all your suggestions but nothing seems to have worked. The urls that the ports need access to are also returning error messages - in particular:

So I guess I have to wait for Sefaira to fix it their end. I have been onto them too. Thanks again for the suggestions!