It looks like I'm having the same problem:
Jun 30, 2021 at 7:01:41 AM
Reloading plugin "Web Server 2021.1.0"
Starting plugin "Web Server 2021.1.0" (pid 684)
Started plugin "Web Server 2021.1.0"
Web Server Starting Indigo Web Server...
Error plugin "Web Server 2021.1.0" (pid 684) unexpectedly stopped -- restarting in 20 seconds
I tried the solution from above (remove/reinstall cryptography) and then rebooted the server and restarted Indigo but I'm still seeing the same errors.
I reran the 2021.1 installer (after stopping the server and the client) but I'm still seeing the same errors.
Is there a way to see a more detailed error message about why the Web Server is crashing?
I'm running on 10.15.7 and v7.5 ran just fine on this server.