I've found debug_printf, which works pretty decent, so no need of cout anymore..
Now I got the line inside CreateFile (part of vfs.cpp) where it crashes:
RETURN_STATUS_IF_ERR(realDirectory->Store(name, fileContents, size));
I've already checked that size is > 0 and name isn't empty. Both got believable values at the error. Have no clue how to check fileContents, though.
Could the crash output (wsysdep.cpp(358): Assertion failed: "charsWritten < max_chars") mean that size was too big for some reason?