Today I’ve been having this problem out of the blue:
The type initializer for 'Sitecore.Install.Serialization.IOUtils' threw an exception.”
Searching for this error in relation to Sitecore usually points to
Newtonsoft.Json.dll as the culprit, but in my case it was
Mvp.Xml.dll, that some NuGet or somesuch upgraded to a higher version thus causing this error. I reverted to the one that shipped with the Sitecore package and everything worked fine again.