From Rainer Volz:

In case anybody has the same problem, I (probably) found the source and a
workaround.

The error “Symbol’s function definition is void: TeX-add-style-hook” is caused
by the footnote-mode, which in turn is loaded by muse-mode, as a default minor
mode in “Muse/Muse Mode/Muse Mode Hook”.

Turning footnote-mode off lets me enter muse-mode (and planner-mode) without
the error. Since I rarely use footnotes I can live without footnote-mode.

If you can’t: several messages found on the net seem to indicate that the real
source of the problem is a path/duplicate file problem between the normal
texinfo.el and a texinfo.el supplied by Auctex. footnote-mode tries to load
the normal texinfo.el but gets the Auctex version, which then tries to run the
function “TeX-add-style-hook”. A solution could be to change the load-path for
Emacs libraries or to deinstall Auctex.

Hope that helps,
Raine

E-Mail from Rainer Volz