ACCOUNT SOLVED: with a new account name. may be a typo in the e-mail
THOUGH OPEN:   where is the admin contact

I tried to create an account on https://portal.sql-contest.isccloud.io/account/login

Just the welcome mail never arrived.
the page proposed to contact admin in this case   

Note: If you do not receive an email, it might be that the username or email address entered is not registered for an account. If you are unable to remember the username or email address you used to sign up for an account, please contact your system administrator for assistance.

No further notice. Who is that system admin? Which mail ?
Not so impressive
 

from docs:

If you are using a UNIX® or Linux system, you can provide a list of alias definitions which the Terminal will set automatically at the start of every session. Define these aliases (one per line) in a file named .iris_init in the home directory.

never tried this. I rarely work on *X

Creating Aliases for Commonly Used Commands

Do ##class(Security.Users).UnExpireUserPasswords("*")
or
set Prop("Password")=NewPassword
set tSC=##class(Security.Users).Modify(Username,.Prop)

Classref1
 Classref2

USER>:?
 :<number>    Recall command # <number>
 :?           Display help
 :py          Do $system.Python.Shell()
 :mdx         Do $system.DeepSee.Shell()
 :sql         Do $system.SQL.Shell()
 :tsql        Do $system.SQL.TSQLShell()
 :alias       Create/display aliases
 :clear       Clear history buffer
 :history     Display command history
 :unalias     Remove aliases

HTH

???  looks like Move to End only works once after Open   not sure after Rewind

instead of %File, I'd suggest to use %Stream.FileCharacter
it has a method MoveToEnd()  that's what might solve your append issue
more

OOPS !  #11 Query as %Query or Query based on ObjectScript

if Sifra is the IDKEY of Material what is the difference
between Material (which is the ID already) and Material->Sifra  ???

While %FromJSON() accepts a String or a character Stream as Input
you have in IRIS also %FromJSONFile() that allows direct input from file
and shortcuts the creation of a Stream.
But for both cases take care of encoding  (I'm burned)
​​​​​​​NOTE: RFC 7159 specifies that the default encoding for JSON values uses UTF-8

??

SQL type TEXT maps correctly to %Stream.GlobalCharacter,
- anyhow this gets not granted on the way from ADO to IRIS-

it would be useful to check if the behavior is the same with a normal ODBC tool.
I tested recently the opposite direction from IRIS to WinSQL and that was OK.

Since transmitting streams always requires some chopping and assembling
at both ends not to break the buffers, it is important if the error persists outside ADO.

just a minor part:
Is it possible while sending an insert/Update to database via the ado driver for iris, that this SQL is executed without creating a cached query?
NO:
In IRIS and also the latest of Caché / Ensemble every query is cached. 
real non-cached queries date back by ~ 10 years. It wasn't just that visible.
 
As you describe it, it seems the LONGVARCHAR or LONGVARBINARY
are not honored correctly when coming from ADO.

pls. Check the related class definition.
And be aware that once created as VARCHAR it can't be changed  
dynamically to LONGVARCHAR  on the fly. This type is a static definition.