I can trigger it reliably using the following skeleton init.el:

(setq savehist-autosave-interval 10)
(savehist-mode)
(setq epg-pinentry-mode 'loopback)

Visiting a test.gpg file and afterwards encrypting it symmetrically
gives the following error:

Debugger entered--Lisp error: (wrong-type-argument listp t)
  savehist-save(t)
  savehist-autosave()
  apply(savehist-autosave nil)
  timer-event-handler([t 23821 9812 623801 10 savehist-autosave nil nil
712000])
  read-string("Confirm password: " nil t nil)
  read-passwd("Confirm password: " nil nil)
  read-passwd("Passphrase for symmetric encryption for /home/drot..." t)
  epa-passphrase-callback-function(#s(epg-context :protocol OpenPGP
:program "/usr/bin/gpg2" :home-directory nil :armor nil :textmode nil
:include-certs nil :cipher-algorithm nil :digest-algorithm nil
:compress-algorithm nil :passphrase-callback
(epa-file-passphrase-callback-function . "/home/drot/test.gpg")
:progress-callback (epa-progress-callback-function . "Encrypting
/home/drot/test.gpg") :edit-callback nil :signers nil :sig-notations nil
:process #<process epg> :output-file "/tmp/epg-outputk8NwaR" :result nil
:operation encrypt :pinentry-mode loopback :error-output "" :error-buffer
#<buffer  *epg-error*>) SYM "/home/drot/test.gpg")
  epa-file-passphrase-callback-function(#s(epg-context :protocol OpenPGP
:program "/usr/bin/gpg2" :home-directory nil :armor nil :textmode nil
:include-certs nil :cipher-algorithm nil :digest-algorithm nil
:compress-algorithm nil :passphrase-callback
(epa-file-passphrase-callback-function . "/home/drot/test.gpg")
:progress-callback (epa-progress-callback-function . "Encrypting
/home/drot/test.gpg") :edit-callback nil :signers nil :sig-notations nil
:process #<process epg> :output-file "/tmp/epg-outputk8NwaR" :result nil
:operation encrypt :pinentry-mode loopback :error-output "" :error-buffer
#<buffer  *epg-error*>) SYM "/home/drot/test.gpg")
  epg--status-GET_HIDDEN(#s(epg-context :protocol OpenPGP :program
"/usr/bin/gpg2" :home-directory nil :armor nil :textmode nil :include-certs
nil :cipher-algorithm nil :digest-algorithm nil :compress-algorithm nil
:passphrase-callback (epa-file-passphrase-callback-function .
"/home/drot/test.gpg") :progress-callback (epa-progress-callback-function .
"Encrypting /home/drot/test.gpg") :edit-callback nil :signers nil
:sig-notations nil :process #<process epg> :output-file
"/tmp/epg-outputk8NwaR" :result nil :operation encrypt :pinentry-mode
loopback :error-output "" :error-buffer #<buffer  *epg-error*>)
"passphrase.enter")
  epg--process-filter(#<process epg> "[GNUPG:] INQUIRE_MAXLEN 100\n[GNUPG:]
GET_HIDDEN pa...")
  accept-process-output(#<process epg> 1)
  epg-wait-for-status(#s(epg-context :protocol OpenPGP :program
"/usr/bin/gpg2" :home-directory nil :armor nil :textmode nil :include-certs
nil :cipher-algorithm nil :digest-algorithm nil :compress-algorithm nil
:passphrase-callback (epa-file-passphrase-callback-function .
"/home/drot/test.gpg") :progress-callback (epa-progress-callback-function .
"Encrypting /home/drot/test.gpg") :edit-callback nil :signers nil
:sig-notations nil :process #<process epg> :output-file
"/tmp/epg-outputk8NwaR" :result nil :operation encrypt :pinentry-mode
loopback :error-output "" :error-buffer #<buffer  *epg-error*>)
("BEGIN_ENCRYPTION"))
  epg-start-encrypt(#s(epg-context :protocol OpenPGP :program
"/usr/bin/gpg2" :home-directory nil :armor nil :textmode nil :include-certs
nil :cipher-algorithm nil :digest-algorithm nil :compress-algorithm nil
:passphrase-callback (epa-file-passphrase-callback-function .
"/home/drot/test.gpg") :progress-callback (epa-progress-callback-function .
"Encrypting /home/drot/test.gpg") :edit-callback nil :signers nil
:sig-notations nil :process #<process epg> :output-file
"/tmp/epg-outputk8NwaR" :result nil :operation encrypt :pinentry-mode
loopback :error-output "" :error-buffer #<buffer  *epg-error*>) #s(epg-data
:file nil :string "hello") nil nil nil)
  epg-encrypt-string(#s(epg-context :protocol OpenPGP :program
"/usr/bin/gpg2" :home-directory nil :armor nil :textmode nil :include-certs
nil :cipher-algorithm nil :digest-algorithm nil :compress-algorithm nil
:passphrase-callback (epa-file-passphrase-callback-function .
"/home/drot/test.gpg") :progress-callback (epa-progress-callback-function .
"Encrypting /home/drot/test.gpg") :edit-callback nil :signers nil
:sig-notations nil :process #<process epg> :output-file
"/tmp/epg-outputk8NwaR" :result nil :operation encrypt :pinentry-mode
loopback :error-output "" :error-buffer #<buffer  *epg-error*>) "hello" nil)
  epa-file-write-region(nil nil "/home/drot/test.gpg" nil t "~/test.gpg"
nil)
  apply(epa-file-write-region (nil nil "/home/drot/test.gpg" nil t
"~/test.gpg" nil))
  epa-file-handler(write-region nil nil "/home/drot/test.gpg" nil t
"~/test.gpg" nil)
  write-region(nil nil "/home/drot/test.gpg" nil t "~/test.gpg")
  basic-save-buffer-2()
  basic-save-buffer-1()
  basic-save-buffer(t)
  save-buffer(1)
  funcall-interactively(save-buffer 1)
  call-interactively(save-buffer nil nil)
  command-execute(save-buffer)

Attempting to quit Emacs after that gives the following error:

Debugger entered--Lisp error: (wrong-type-argument listp t)
  savehist-save(t)
  savehist-autosave()
  kill-emacs()
  save-buffers-kill-emacs(nil)
  save-buffers-kill-terminal(nil)
  funcall-interactively(save-buffers-kill-terminal nil)
  call-interactively(save-buffers-kill-terminal nil nil)
  command-execute(save-buffers-kill-terminal)

These errors start to happen every time there's a minibuffer prompt for GPG
decryption.

In GNU Emacs 27.0.50 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.5,
cairo version 1.16.0)
 of 2019-06-21 built on lambda
Repository revision: 869cf5cbc0721bbe9e4df557f4ed3633fb13c05f
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12004000
System Description: Debian GNU/Linux 10 (buster)