Sorry to reply late.

I could not patch your patch file on my Emacs 26.1 tramp.el. I guess I 
will wait until I get time to have Emacs 26.2 installed and test your 
change again.

Wanrong

On 5/26/2019 3:50 PM, Michael Albinus wrote:
> Wanrong Lin <wrglin@gmail.com> writes:
>
>> Hi Michael:
> Hi Wanrong,
>
> [Pls keep the Cc, for the archives]
>
>> I just tested your patch (on Emacs 26.1 as I don't yet have a working
>> Emacs 26.2 set up) , and unfortunately it did not seem to fix the
>> issue. Would you mind taking a look at it again? Thanks.
> Well, I have hard times to reproduce your problem. Anyway, could you try
> the appended patch instead?
>
>> Wanrong
> Best regards, Michael.
>
>
> *** /tmp/ediff16zXeN	2019-05-26 21:45:35.446178244 +0200
> --- /net/ford/albinus/src/tramp/lisp/tramp.el	2019-05-26 21:44:57.446575193 +0200
> ***************
> *** 4676,4681 ****
> --- 4676,4682 ----
>    PROGRAM is nil is trapped also, returning 1.  Furthermore, traces
>    are written with verbosity of 6."
>      (let ((default-directory (tramp-compat-temporary-file-directory))
> + 	(process-environment (default-toplevel-value 'process-environment))
>    	(destination (if (eq destination t) (current-buffer) destination))
>    	(vec (or vec (car tramp-current-connection)))
>    	output error result)
> ***************
> *** 4709,4714 ****
> --- 4710,4716 ----
>    PROGRAM is nil is trapped also, returning 1.  Furthermore, traces
>    are written with verbosity of 6."
>      (let ((default-directory (tramp-compat-temporary-file-directory))
> + 	(process-environment (default-toplevel-value 'process-environment))
>    	(buffer (if (eq buffer t) (current-buffer) buffer))
>    	result)
>        (tramp-message
> ***************
> *** 4738,4743 ****
> --- 4740,4746 ----
>    If an error occurs, it returns nil.  Traces are written with
>    verbosity of 6."
>      (let ((default-directory (tramp-compat-temporary-file-directory))
> + 	(process-environment (default-toplevel-value 'process-environment))
>    	(vec (or vec (car tramp-current-connection)))
>    	result)
>        (if args