EPiServer.XmlSerializers doesn't exist. Sounds like you have some wrong bindings configured in web.config.
That exception is actually thrown by .NET. When a XmlSerializer is used it generates assemblies during serialization/deserialization it also tries to load those generated assemblies and if no such exist that exception is thrown. That is actually an expected behaviour and is caught (you only see it if you have break at all exceptions tured on).
Personally I think they should have implemented it differently so those exceptions are not thrown.
see for example:
I am currently upgrading a site from 4.5 to 6R2. The database has been upgraded and I am working on the source code.
But when I build and run the site I get the following error in the global.asax:
Could not load file or assembly 'EPiServer.XmlSerializers, Version=6.1.379.0, Culture=neutral, PublicKeyToken=8fe83dea738b45b7' or one of its dependencies. The system cannot find the file specified.
I have not been able to find anything at all on that on the web or through reflection (though I have not been through ALL episerver dll's)
Any ideas out there?