This is a known issue, resulting from updating to current firmware from legacy with Keep settings enabled.
You will either need to reset the device to factory defaults or login to router via SSH and make some modifications in order to workaround it. For options, on how to SSH into your device, refer here.
Next you will need to execute the following command, to edit a certain configuration file:
Press letter i to enable editing and replace the following lines:
- list listen_http '80'
- list listen_https '443'
- option listen_http '80'
- option listen_https '443'
Once done press Esc, then enter :wq and press Enter.
Lastly, restart the service with /etc/init.d/uhttpd restart command.
This should allow you to save.