mm-inline-media-tests contains lambda's, and itʼs byte-compiled, so
when you customize its value, byte-code is saved in your .emacs. One
of its components looks like this:

     ("text/x-vcard" mm-inline-text-vcard
      #[257 "\300\301!\206	%0\302\303!\207"
            [featurep vcard locate-library "vcard"]
            3 "

     (fn HANDLE)"])

That has an embedded 'NUL' just before the \302 (Iʼve encoded it),
which causes 'buffer-file-coding-system' to be incorrect
('no-conversion in this case) so any non-ascii character causes a read
error:

$ HOME=/tmp emacs --debug-init

Debugger entered--Lisp error: (invalid-read-syntax "?" 1 0)
  read(#<buffer  *load*>)
  eval-buffer(#<buffer  *load*> nil "/tmp/.emacs" nil t)  ; Reading at buffer position 573
  load-with-code-conversion("/tmp/.emacs" "/tmp/.emacs" t t)
  load("~/.emacs" noerror nomessage)

Sample .emacs attached. There are probably other defcustom's with the
same issue, I haven't checked.