This looks like a problem in the connection between web UI and server, so I don’t think the backup size or backend has anything to do with it.
Do the crashes only occur when you actively use the UI? In that case:
The invalid port error might be caused by an incorrectly formatted URL where colons are not escaped properly, or it might miss a forward slash after the port number. That would be an error in the JavaScript of the UI. This would be a request visible in your browser, so you could check in your browser debugging tools (in the network tab) what requests are made right before the error occurs.
If you can reproduce the error somewhat often, you should open the browser debugger before you run the backup that might trigger the problem. Either way, there shouldn’t be any problems for scheduled backups since they don’t involve the UI.
Do these errors also occur while you are not using the UI (there are 500+ restarts in the log):
Another reason might be that your server is publicly accessible, so anyone might make some requests to test out the limits of the system. It seems there is some request that can crash the simple http server used by Duplicati.
In that case, I would strongly suggest that you use a VPN and isolate the duplicati port from the public Internet, or use SSH port forwarding when you need to access the UI. I would not trust the security of the Webserver on a public port, it is pretty outdated.