The Visio cache is located on your client PC and indeed allows you to load Visual Overviews without the server needing to send the latest version every time AND without cube needing to parse it again. It contains a serialized version of the document object as it was parsed by cube along with some additional info, such as a file version number (see below) and when the file was last changed.

When a new file is uploaded by yourself or the server notifies the client that a visio file has changed, the cached file is removed so that the new one can be loaded (and also cached).

An outlier reason the file is retrieved from the server instead of from the cache is when cube's file version number doesn't match with what was deserialized from the cache. This is most often due to new shape data being added in a new version, as these are part of serialization.

Getting a message that states (de)serialization has failed means exactly what it says. Somewhere during (de)serialization a problem occurred. Most often this will be due to a failure in reading/writing the cache file. The exception included along with the message should tell you more.