I was waiting for it.
And I missed this setup in other places.

develop on prod ?  

you are right on %objlasterror.  and that's how I work it around
but if you want to keep a log and chase sporadic error events this is quite an extra effort.  

definitely a good idea to run routine ^%GSIZE  to find the big consumers and packing
directory: c:\intersystems\cache\mgr\user\
Page: 1                           GLOBAL SIZE                        08 Aug 2017
                                                                        10:28 AM
      Global        Blocks       Bytes Used  Packing   Contig.
      --------    --------  ---------------  -------   -------
      CacheStd           1              140      2 %         0
      CacheStdS          1               92      1 %         0
      CacheStream      109          764.376     86 %        70
      ERRORS             1               12      0 %         0
      G1                 1               72      1 %         0

thx.
so you confirmed to me that it was't just wishful thinking

as suggested earlier 

congratualtions !

indeed this is surprising.

but is this really:   do content.ReadLine(,,isEOL)

it should be a pass by reference to receive something back do content.ReadLine(,,.isEOL) 
with the <dot> in front of the variable 

You still have the option to create your own %ZLANGC00.int 
and make you own ZZEXIT to avoid the risky HALT
It's a miracle to me how HALT could survive from PDP-7 sn#103 at MGH and nobody touched it.
                           

to me this looks like your content had an extra $C(13,10) at the end.
It could help to  have the full content at hands. 
eg:
before  content.OutputToDevice()
set ck=content.Read()
set ^ck($i(^ck))=ck
do content.Rewind()

I expect  zwrite ^ck will show more than 3  lines
That would indicate that the source of trouble is on the input side. 
Some closing action ?   

I don't see a call for  wnl()   
but ancient coding practices  would suggest its:  writenewline just that
and that's what it does without any other content.
though the remark is rater direct if interpreting feedback := line

// Should be smart as well: routines ending with write ! will create an empty feedback!

just a guess:
with do content.MoveTo(content.Size - 1)   you are probably just between CR and LF 

Could you try do content.MoveTo(content.Size - 3)  just to make sure you a e before CRLF

I'm not sure how EOL is triggered
eventually also check returned success code

For me there is the InterSystems Ensemble Logo - NOW