The message

Parse Error, no assembly associated with Xml key a1:http://schemas.microsoft.com/clr/nsassem/MyProject/MyProject%2C%20Version%3D1.0.0.0%2C%20Culture%3Dneutral%2C%20PublicKeyToken%3Dnull

indicates that the data has been serialized with an assembly that has a PublicKeyToken set to null. This means that at serialization time, this assembly had no strong name.

Now, if you recompile this assembly sign it using a strong name, and redeploy it inplace without chaning the serialized data, the assembly identity will not be considered the same, and the deserialization code will not be able to use it for deserialization, hence the error.

Assemblies used for serialization and deserialization must have the same full name (at least with the formatter and the binder you use).

So it can be a deployment issue (make sure serializing & deserializing assemblies have the same full name), or if you really need to be able to deal with assemblies with different full name (but that's unusual), you can use a custom binder, like what's done here: Deserializing data into a dynamically loaded Assembly although I would not recommend it for standard operations.