In Emacs 27.1 on Windows with auctex 13.0.4, I have installed aspell as described in
https://emacs.stackexchange.com/a/45752 and it works, except
in .tex files.

When I M-x ispell-buffer emacs hangs after a few moments and it doesn't
become active before I press C-g and skips the spelling.
The same happens for flyspell.

It seems to work if line 4084 of ispell.el is commented:
      ;;     (ispell-send-string "+\n")    ; set ispell mode to tex


In GNU Emacs 27.1 (build 1, x86_64-w64-mingw32)
of 2020-08-21 built on CIRROCUMULUS
Repository revision: 86d8d76aa36037184db0b2897c434cdaab1a9ae8
Repository branch: HEAD
Windowing system distributor 'Microsoft Corp.', version 10.0.17134
System Description: Microsoft Windows 10 Enterprise (v10.0.1803.17134.2087)

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user-error: No further undo information
apply: Command attempted to use minibuffer while in minibuffer

Configured using:
'configure --without-dbus --host=x86_64-w64-mingw32
--without-compress-install 'CFLAGS=-O2 -static''

Configured features:
XPM JPEG TIFF GIF PNG RSVG SOUND NOTIFY W32NOTIFY ACL GNUTLS LIBXML2
HARFBUZZ ZLIB TOOLKIT_SCROLL_BARS MODULES THREADS JSON PDUMPER LCMS2 GMP

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Major mode: BibTeX

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Load-path shadows:
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Features:
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