Hi Lars,
I have had the same problem, only mine started as soon as I paid for a Standard Edition license after using the free trial. The standard edition does not use the template cache whereas the professional one does. This means that for s standard edition license, the 'Template Cache' directory is not present in the 'Application Data\CodeSmith\v4.1' directory (in fact it is even deleted if you create it manually).
I have raised a 'premium support' call and will post the answer here when I get it.
System.IO.DirectoryNotFoundException: Could not find a part of the path 'C:\Documents and Settings\alex.brooker\Application Data\CodeSmith\v4.1\Template Cache\NetTiers_cst.cs'.
at System.IO.__Error.WinIOError(Int32 errorCode, String maybeFullPath)
at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, Int32 rights, Boolean useRights, FileShare share, Int32 bufferSize, FileOptions options, SECURITY_ATTRIBUTES secAttrs, String msgPath, Boolean bFromProxy)
at System.IO.FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options)
at System.IO.StreamWriter.CreateFile(String path, Boolean append)
at System.IO.StreamWriter..ctor(String path, Boolean append, Encoding encoding, Int32 bufferSize)
at System.IO.StreamWriter..ctor(String path)
at CodeSmith.Engine.CodeTemplateCompiler.Compile(String outputPath, Boolean forceRecompile)
at CodeSmith.Gui.CodeTemplateGenerator.a(String A_0, String A_1, String A_2, Boolean A_3, Boolean A_4)
cheers,
Alex