On Fri, 26 Oct 2018 20:21:11 +0200
"Michael K." <michael@???> wrote:

> A "text editor" tells me on opening the file: this file is a invalid
> UTF-8

I also use Geany. I checked my own ".xsession-errors" and mine opens
correctly. Since the contents of this file are individual, I wonder if
you have something that is putting unusual characters into that file.

I see some other good ideas in this thread; you can try another
editor.. however, maybe someone can help with a good way to track down
exactly what parts of a file would have those interesting characters.

Consider copying the file into a backup, deleting it, and going about
your day for a little while. Then check into that file to see if it's
still invalid.

  - If it is, then you'll at least have a smaller file to examine.
  - If it isn't, then maybe there are things you only rarely do which
    is causing such a problem.  Or maybe the problem will go away and
    never return?  :)