Failed to load viewstate. The control tree into which viewstate is being loaded

I am receiving the following error message after an HTTP POST on an ASP.NET form hosted inside a UserControl:

Failed to load viewstate. The control tree into which viewstate is being loaded must match the control tree that was used to save viewstate during the previous request. For example, when adding controls dynamically, the controls added during a post-back must match the type and position of the controls added during the initial request.

Here's additional info:

• I'm running .NET 4.5 RC
• It's an Umbraco 4.7-based website
• On my local dev machine the form works perfectly
• This error only occurs on the staging server which has .NET 4.5 (only), MSSQL 2012 Express, IIS 7.5, Windows 7 (I know, it's not a real server yet, one day maybe...)
• The server is not part of a web farm (or garden, tho that should be irrevelant)
• The user control does render controls dynamically

I have applied all the latest service packs. I have run out of ideas now! I have even restarted it and also performed a richual over the server involving a song and a special dance to no avail.

OK, so the answer is literally: "Set up a new server with all the same software as the last one and try again" and it works now.