I was trying to install a custom extension for ProGet and got this error message:

System.Runtime.Serialization.SerializationException: There was an error deserializing the object of type Inedo.Extensibility.UpackMetadata. Encountered unexpected character 'ï'. ---> System.Xml.XmlException: Encountered unexpected character 'ï'.
at
...

Turns out that visual studio used UTF-8 with BOM to encode my upack.json file which caused this error.

Posting this to help others who might also run into this issue.