‡" [gnus-summary-buffer gnus-summary-select-article nil article gnus-summary-remove-process-mark] 5)
  gnus-inews-yank-articles((252 253 254))
  gnus-summary-reply((252 253 254) nil)
  gnus-summary-reply-with-original(nil)
  call-interactively(gnus-summary-reply-with-original)

Normally, when replying, the window is resized, but I am not
sure if this occurs before or after this error.

I suspect this section of my config file may also
be relevant (copied from somewhere, perhaps the Info doc):

     (setq gnus-use-trees t
           gnus-generate-tree-function 'gnus-generate-horizontal-tree
           gnus-tree-minimize-window nil)
     (gnus-add-configuration
      '(article
        (vertical 1.0
                  (horizontal 0.25
                              (summary 0.75 point)
                              (tree 1.0))
                  (article 1.0))))



PS, when sending this message, I got the following warning:

Warning: Your message contains 5 parts.  Really send? (y or n)  

5 parts??? That was before I had any MIME attachments.

I said n

then I got the message

Edit your message to use only one charset

So, I will attach another copy of that buffer, in case the first
one doesn't work: