When using the YAMLGenerator in the MINIMIZE_QUOTES mode, the generator serializes strings in plain style, i.e. without quoting them. However, as stated in the YAML Spec such plain style strings cannot contain some special characters such as ':', ',', and others.

As with the serialization of boolean-like strings, the YAMLGenerator should be adjusted to always quote strings containing those special characters.

I will shortly try to provide a pull request to fix this issue.