Elastic Beanstalk configuration error: invalid postgres engine version

Hi,

I've had my elastic beanstalk configuration running without problems for quite some time. I wanted to make a change in my configuration to upgrade the EC2 server, but the UI is stuck with the following message displayed at the top of the screen:

Configuration validation exception: Invalid option value: '12.8' (Namespace: 'aws:rds:dbinstance', OptionName: 'DBEngineVersion'): Engine Version 12.8 not supported for postgres db (2) 

It seems that the form values are not set properly, and as a result i cannot see/change/select anything in my configuration.

I also checked the console and found the following client side error:

Uncaught (in promise) InvalidParameterValueException: com.amazon.coral.service.InvalidParameterValueException: Cannot find version 12.8 for postgres

I tried setting the config to my actual database engine value with eb config:

aws:rds:dbinstance:
  HasCoupledDatabase: 'false'
  DBEngineVersion: '12.11'

But getting the following error:

ERROR: ServiceError - Configuration validation exception: RDS DB Engine Version option setting is not allowed to changed

EDIT: Also tried setting this in .ebextensions, but although the deployment succeeds, I am getting the same error on Elastic Beanstalk console:

option_settings:
  aws:rds:dbinstance:
    DBEngineVersion: 12.11

Help is very much appreciated so I can upgrade my EC2 instance.