(Hopefully someone else can reproduce this issue without my having to
 start an uncustomised Gnus.)

1. M-x toggle-debug-on-error RET
2. M-x set-variable RET mm-text-html-renderer RET nil RET
3. Open a HTML article in Gnus, e.g. by typing RET in a summary buffer

This results in the attached backtrace.

I'm quite unfamiliar with the relevant code, but the patch I am
currently using as a workaround follows my signature, as do listings of
my Gnus and Emacs versions.

Thanks,

-- 
Basil

Gnus v5.13
GNU Emacs 27.0.50 (build 16, x86_64-pc-linux-gnu, X toolkit, Xaw3d scroll bars)
 of 2018-03-19