On 2016-08-06 07:21, Eli Zaretskii wrote:
>> From: Johan Bockgård <bojohan@gnu.org>
>> Cc: Clément Pit--Claudel <clement.pit@gmail.com>,
>>   emacs-devel@gnu.org
>> Date: Sat, 06 Aug 2016 13:17:07 +0200
>>
>> Eli Zaretskii <eliz@gnu.org> writes:
>>
>>>> I've attached emacsclient-input. Can someone help me figure what's
>>>> going wrong? The error doesn't appear if I modify any of the
>>>> backslashes, dashes, or newlines in the input...
>>>
>>> Put a breakpoint in Fsignal, and see who calls it and why.
>>
>> The error comes from the emacsclient side:
>>
>> $ git grep 'Unknown message'
>> lib-src/emacsclient.c:        printf ("*ERROR*: Unknown message: %s\n", p);
> 
> Thanks.  What is the message received from the server that triggers
> this?

The client print 

    *ERROR*: Unknown message: E EE EEEEE\n"

Is that the information you were looking for?