Copying here my message from the vim-dev ml:
Hm, here:

,----[ quickfix.c ]-
| 3428 #ifdef FEAT_AUTOCMD
| 3429 ++curbuf_lock;
| 3430 #endif
| 3431 set_option_value((char_u *)"ft", 0L, (char_u )"qf", OPT_LOCAL);
| 3432 curbuf->b_p_ma = FALSE;
| 3433
| 3434 #ifdef FEAT_AUTOCMD
| 3435 keep_filetype = TRUE; / don't detect 'filetype' */
`----

we are setting curbuf_lock. set_option_value() triggers the FileType
autocommand, and will finally call do_one_cmd() to execute the :resize
command. However in do_one_cmd() we do check curbuf_lock before
continuing:

,----[ ex_docmd.c ]-
| 2463 #ifdef FEAT_AUTOCMD
| 2464 /* Disallow editing another buffer when "curbuf_lock" is set.
| 2465 * Do allow ":edit" (check for argument later).
| 2466 * Do allow ":checktime" (it's postponed). */
| 2467 if (!(ea.argt & CMDWIN)
| 2468 && ea.cmdidx != CMD_edit
| 2469 && ea.cmdidx != CMD_checktime
| 2470 && !IS_USER_CMDIDX(ea.cmdidx)
| 2471 && curbuf_locked())
| 2472 goto doend;
| 2473 #endif
`----

However it is not clear to me, why there is the check to curbuf_locked()
there, because as you say, there is no indication that we are going to
move to another buffer. Perhaps that should be changed to this
condition:

diff --git a/src/ex_docmd.c b/src/ex_docmd.c
index bdd152dfd..a7946d872 100644
--- a/src/ex_docmd.c
+++ b/src/ex_docmd.c
@@ -2468,7 +2468,7 @@ do_one_cmd(
                && ea.cmdidx != CMD_edit
                && ea.cmdidx != CMD_checktime
                && !IS_USER_CMDIDX(ea.cmdidx)
-               && curbuf_locked())
+               && (ea.addr_type && curbuf_locked()))
            goto doend;
 #endif

(Well, this seems to fix your case, while still preventing e.g.
au Filetype qf wincmd p)

Best,
Christian

Hello. I configure as moving help window by autocmd FileType help if &l:buftype ==# 'help' | wincmd K | endif. It worked with no errors before but emits E788 error recently. I believe my issue is related to this.
Ken Hamada

I also have the same [new] E788 problem recently and still in 8.0.702 with this -

au FileType qf call AdjustWindowHeight(5, 10)
function! AdjustWindowHeight(minheight, maxheight)
    let l = 1
    let n_lines = 0
    let w_width = winwidth(0)
    while l <= line('$')
        " number to float for division
        let l_len = strlen(getline(l)) + 0.0
        let line_width = l_len/w_width
        let n_lines += float2nr(ceil(line_width))
        let l += 2
    endw
    exe max([min([n_lines, a:maxheight]), a:minheight]) . "wincmd _"
endfunction

This ex_docmd.c change (from above):

-               && curbuf_locked())
+               && (ea.addr_type && curbuf_locked()))

resolves it.