This appears to be similar to a known issue, which is planned to be resolved in 7.5 firmware version.
Device reset to factory defaults should resolve it, as it is likely caused by a firmware update with Keep settings option enabled.
Otherwise, to solve this without losing current configuration, you will need to login to the router via SSH. 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.