Hi,

Possible errors -

1. Check if by mistake you got a  white spaces 

    UseDevelopmentStorage=true;DevelopmentStorageProxyUri=https://127.0.0.1 instead of UseDevelopmentStorage=true; DevelopmentStorageProxyUri=https://127.0.0.1 

2. Don't forget to include the "https://" or "https://" when using the proxyUri. Like: 

    UseDevelopmentStorage=true;DevelopmentStorageProxyUri=https://127.0.0.1 

3. Some time newline character (\r\n)  comes in the connection string which we by mistake ignore

Please check these

Thanks

Sourav