> Can you provide all the necessary files and a procedure to reproduce
> this problem?  Given that the problem is so elusive, I think having a
> recipe to reproduce exactly what you do and with the same data will
> allow us to investigate much more efficiently.

I’ve uploaded the repro files at
http://users.itk.ppke.hu/~marma/downloads/emacsdebug.zip

The procedure is:
1. Create a Windows user with an accented letter in its name (mine is “Márton”)
2. Unzip the above files to %APPDATA% (C:\Users\Márton\AppData\Roaming)
3. Start Emacs so that the working directory has no accented letter in the path
4. Notice how in the Messages buffer, the path name is wrongly encoded.