Some more information ..

Earlier "emacsclient -c&" used to work. Now doing that auto-suspends that
process and on doing fg, I get "broken pipe".

But "emacsclient -c" seems to be working (without that &). But then I need
to block a new terminal for each new emacsclient invocation (with or
without -c) ..

Also, I fixed a regression in one of the 3rd party Elisp packages that got
updated in last month.

After that, instead of "broken pipe" error, I now see "*ERROR*: Assertion
failed: (eq (match-end 0) (length string))"

That error is originating from servel.el:

=====
  (condition-case err
      (progn
(server-add-client proc)
;; Send our pid
(server-send-string proc (concat "-emacs-pid "
(number-to-string (emacs-pid)) "\n"))
(if (not (string-match "\n" string))
            ;; Save for later any partial line that remains.
            (when (> (length string) 0)
              (process-put proc 'previous-string string))

          ;; In earlier versions of server.el (where we used an
`emacsserver'
          ;; process), there could be multiple lines.  Nowadays this is not
          ;; supported any more.
          (cl-assert (eq (match-end 0) (length string)))
=====

I get that assertion error if I do "emacsclient -c&". But if I do just
"emacsclient -c", a new Emacsclient frame launches fine.