Hello, I think you are trying to set this parameter with a wrong format … but usually you don’t really need to set this parameter and It’s enough to enable periodic informs and set the inform interval:
InternetGatewayDevice.ManagementServer.PeriodicInformEnable: true
InternetGatewayDevice.ManagementServer.PeriodicInformInterval: no of seconds
To also set the PeriodicInformTime, try with the following format:
2020-05-28T16:00:05 or 2020-05-28T16:00:05.123456
You must have a preset (maybe with a provision script attached) that is trying to set this parameters. When you find the line, comment it or update the format and the error will go away.