Subject: bug#31194: 26.1; `display-buffer-in-major-side-window' was renamed without an alias.

From: Glenn Morris <[email-masked]>
Date: Tue, 17 Apr 2018 11:51:35 -0400

Phil Sainty wrote:

> I have at least one library installed (which-key.el) which triggers an
> error on this account.

Sounds like you need to update; ref https://debbugs.gnu.org/24828#19 .