We use a Silverlight OOB application that prints orders and invoices, usually writing them to disk as a pdf with utilities such as dopdf.
Recently this stopped working. All virtual printer drivers that access the disk report an access violation - even if the directory is fully accessible by "everyone".
I suspect this to have something to do with the latest Silverlight Release 5.1.40416.0 from May, 12 this year - because I know it used to work and Microsoft's release history talks about "Vulnerability in Silverlight Could Allow Elevation of Privilege".
EDIT: There is another symptom unrelated to printing. It used to be possible to open mail programs by clicking on mailto-Hyperlinkbuttons. That too, has since become broken except in the circumstances named in my answer.
%temp%\low
exist and have the correct permissions in each user's profile directory? (usually expands to something like `C:\Users\UserName\AppData\Local\Temp\Low`).